Method of applying a sealant to an aircraft component

ABSTRACT

The method includes applying a curable sealant to a surface of the aircraft component, forming a non-tacky skin on an exposed portion of the curable sealant within a first time period while allowing a portion of the curable sealant adjacent the surface of the aircraft component to be liquid for a second time period. The second time period is at least twice the first time period. The curable sealant comprises at least one of an adhesion promoter or a wetting agent.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application Nos.62/416,948 and 62/416,970, filed Nov. 3, 2016, the disclosure of whichis incorporated by reference in its entirety herein.

BACKGROUND

Sulfur-containing polymers are known to be well-suited for use inaerospace sealants due to their fuel resistant nature upon crosslinkingSuch crosslinking can be carried out, for example, by reaction of athiol-terminated sulfur-containing compound with an epoxy resin,generally in the presence of an amine accelerator as described in U.S.Pat. No. 5,912,319 (Zook et al.). A desirable combination of propertiesfor aerospace sealants, which is difficult to obtain, is the combinationof long application time (i.e., the time during which the sealantremains usable) and short curing time (the time required to reach apredetermined strength).

Other crosslinked sulfur-containing polymers have been made, forexample, by reaction of a thiol-terminated sulfur-containing compoundwith a polyene in the presence of a photoinitiator as described in U.S.Pat. Appl. Nos. 2012/0040103 (Keledjian et al.) and 2016/0032058 (Ye etal.).

SUMMARY

When photochemical curing thiol-terminated sulfur-containing compoundsto make sealants, it has been generally considered desirable to achievefull cure as quickly as possible. In contrast, the present disclosurerelates to a method of applying aerospace sealants to form a durable,tack-free surface skin but remain un-gelled beneath the crosslinkedskin. This method offers the advantage of providing a material having aFOD (foreign object debris) free surface while leaving a portion of theapplied sealant un-gelled so that smaller molecules, such as wettingagents and adhesion promoters, are free to migrate to the interfacebetween the sealant and the aircraft component. The method of thepresent disclosure can allow manufacturers to more quickly assembleaircraft by reducing time wasted waiting for sealant to becometack-free.

In one aspect, the present disclosure provides a method of applying asealant to an aircraft component. The method includes applying a curablesealant including at least one of an adhesion promoter or wetting agentto a surface of the aircraft component, forming a non-tacky skin on anexposed portion of the curable sealant within a first time period whileallowing a portion of the curable sealant adjacent the surface of theaircraft component to be liquid for a second time period. The secondtime period is at least twice the first time period.

In some embodiments, the method according to the present discloseincludes providing a composition comprising a polythiol comprising morethan one thiol group and a curing agent comprising more than onecrosslinking group. A solution including a photoinitiator is thenapplied to the surface of the composition. Following the application ofthe solution including the photoinitiator to the surface of thecomposition, a non-tacky skin can be made on the surface by exposing thecomposition to an appropriate light source. In other embodiments inwhich the curable sealant includes a photoinitiator, the light source,power level, temperature, and presence of fillers, for example, can betuned to achieve a non-tacky skin while allowing portion of the curablesealant adjacent the surface of the aircraft component to remain liquidfor a desirable time period. In still other embodiments, the curablesealant includes at least one of an oxygen-activated curing agent or amoisture-activated curing agent. When such curing agents are exposed tooxygen or moisture, respectively, non-tacky skin can be formed whileallowing portion of the curable sealant adjacent the surface of theaircraft component to remain liquid for a desirable time period.

In this application:

Terms such as “a”, “an” and “the” are not intended to refer to only asingular entity, but include the general class of which a specificexample may be used for illustration. The terms “a”, “an”, and “the” areused interchangeably with the term “at least one”.

The phrase “comprises at least one of” followed by a list refers tocomprising any one of the items in the list and any combination of twoor more items in the list. The phrase “at least one of” followed by alist refers to any one of the items in the list or any combination oftwo or more items in the list.

The terms “cure” and “curable” refer to joining polymer chains togetherby covalent chemical bonds, usually via crosslinking molecules orgroups, to form a network polymer. Therefore, in this disclosure theterms “cured” and “crosslinked” may be used interchangeably. A cured orcrosslinked polymer is generally characterized by insolubility, but maybe swellable in the presence of an appropriate solvent.

The term “liquid” refers to being able to flow at ambient temperature.In some embodiments, the term “liquid” refers to being un-gelled and/orstill having remaining open time. In some embodiments, the term “liquid”refers to having sufficient flow in order to wet out a surface whenmanually spread with a spatula at 21° C.

The term “polymer or polymeric” will be understood to include polymers,copolymers (e.g., polymers formed using two or more different monomers),oligomers or monomers that can form polymers, and combinations thereof,as well as polymers, oligomers, monomers, or copolymers that can beblended.

“Alkyl group” and the prefix “alk-” are inclusive of both straight chainand branched chain groups and of cyclic groups. In some embodiments,alkyl groups have up to 30 carbons (in some embodiments, up to 20, 15,12, 10, 8, 7, 6, or 5 carbons) unless otherwise specified. Cyclic groupscan be monocyclic or polycyclic and, in some embodiments, have from 3 to10 ring carbon atoms. Terminal “alkenyl” groups have at least 3 carbonatoms.

“Alkylene” is the multivalent (e.g., divalent or trivalent) form of the“alkyl” groups defined above. “Arylalkylene” refers to an “alkylene”moiety to which an aryl group is attached. “Alkylarylene” refers to an“arylene” moiety to which an alkyl group is attached.

The terms “aryl” and “arylene” as used herein include carbocyclicaromatic rings or ring systems, for example, having 1, 2, or 3 rings andoptionally containing at least one heteroatom (e.g., O, S, or N) in thering optionally substituted by up to five substituents including one ormore alkyl groups having up to 4 carbon atoms (e.g., methyl or ethyl),alkoxy having up to 4 carbon atoms, halo (i.e., fluoro, chloro, bromo oriodo), hydroxy, cyano, or nitro groups. Examples of aryl groups includephenyl, naphthyl, biphenyl, fluorenyl as well as furyl, thienyl,pyridyl, quinolinyl, isoquinolinyl, indolyl, isoindolyl, triazolyl,pyrrolyl, tetrazolyl, imidazolyl, pyrazolyl, oxazolyl, and thiazolyl.

All numerical ranges are inclusive of their endpoints and non-integralvalues between the endpoints unless otherwise stated (e.g., 1 to 5includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).

DETAILED DESCRIPTION

Existing sealant products now in use in the aircraft industry aretypically either two-part products or one-part products. For thetwo-part products, once the user mixes the two parts, the reactionbegins and the sealant starts to form into an elastomeric solid. Aftermixing, the time that the sealant remains usable is called theapplication life or open time. Throughout the application life,viscosity of the sealant gradually increases until the sealant is tooviscous to be applied. Application life and cure time are typicallyrelated in that short-application-life products cure quickly.Conversely, long-application-life products cure slowly. In practice,customers choose products with differing application lives and curetimes depending on the specific application. This requires the customerto maintain inventories of multiple products to address the productionflow requirements of building and repairing aircraft. For one-partproducts, users can avoid a complicated mixing step, but the product hasto be shipped and stored in a freezer before application. Thus thereexists a need for an aircraft sealing material that can simultaneouslysatisfy the need of having a long application life but can also be curedon demand by the customer.

With existing materials, sealant applicators mix the first and secondcomponents as described above and then apply the mixed sealant to thearea to be sealed. The sealant is tooled to ensure the shape of thejoint meets the desired geometry and so the sealant is free of voids,air pockets, pinholes and other possible defects. The sealed area mustthen be protected for an extended period of time to allow the sealant tobecome tack and FOD-free and hard enough to be durable so thatadditional work operations can be performed in the vicinity of thesealed structure without damaging the applied sealant. Customerspecifications establish maximum allowable times for each of theseapplication characteristics, for example SAE AMS-S-8802 requires a 2hour application life sealant to be tack-free in 40 hours and cured to30 Share A within 72 hours. Aircraft manufacturers use a variety ofmethods to eliminate this waiting time including the use of energyintensive forced air heaters, detackifier products which potertiallydisrupt adhesion of other materials or dissolve in fuel creatingparticles that could clog filters, and the erection of physical barrierssuch as terts to protect the uncured and tacky sealant.

The performance properties of aircraft sealing materials may rely on theability of small molecules to surface segregate. Principle among theseproperties is adhesion. Adhesion with sealant materials is mostgenerally created using one or more wetting agents and/or adhesionpromoters. We have observed that, unlike application life and cure time,the time required for a sealant to adhere to a given substrate isinversely related to cure time. That is, materials that cure through tothe substrate rapidly require longer to develop adhesion to thesubstrate. Rapid gelation and network formation inhibits the mobility ofsmall molecules within the cured matrix thus extending the time requiredfor adhesion to form between the sealant and substrate. The presentdisclosure addresses an unmet need for materials capable of rapidlyforming a tough and durable FOD-free skin and also adhering well to thesubstrate. The method according to the present disclosure allows thesealant to remain liquid beneath that skin so that wetting agents andadhesion promoters are able to migrate to the substrate before thecurable sealant gels.

In the method of the present disclosure, a curable sealant including atleast one of an adhesion promoter or wetting agent is applied to asurface of the aircraft component. A non-tacky skin is formed on anexposed portion of the curable sealant within a first time period, whilea portion of the curable sealant adjacent the surface of the aircraftcomponent is allowed to remain liquid for a second time period. Anon-tacky surface may be one in which the surface no longer tightlyadheres to L-LP-690 standard low density polyethylene film as determinedusing ASTM C₆₇₉. A non-tacky surface may also be one that is FOD-freeaccording to the following evaluation. After curing, fine aluminumshavings can be spread on to the cured sealant surface and allowed toremain undisturbed for 30 seconds at 70° F. (21.1° C.). The sealant canthen be inverted to allow the shavings to fall off, after which thesealant surface can be gently brushed using a fine fiber paintbrush toremove any remaining aluminum shavings. The surface can be consideredFOD-free, akin to non-tacky, if no aluminum shavings remain on thesurface after inversion and/or after brushing.

In some embodiments, the first time period is up to four hours. In someembodiments, the first time period is up to three hours, two hours, onehour, 45 minutes, 30 minutes, 20 minutes, 15 minutes, ten minutes, orfive minutes. In embodiments, for example, in which actinic radiation isused to form a non-tacky skin on the exposed surface of the curablesealant, the first time period can be up to 60, 45, 30, 25, 20, 15, orten seconds. The second time period is at least twice the first timeperiod. The second time period is generally sufficient to allow for atleast one of the adhesion promoter or wetting agent to interact with thesurface of the aircraft component. In some embodiments, the second timeperiod is at least three, five, ten, 20, 25, 50, or 100 times the firsttime period. In embodiments, for example, in which actinic radiation isused to form a non-tacky skin on the exposed surface of the curablesealant, the second time period can be at least 200, 500, 1000, 2000, oreven at least 5000 times the first time period. The first time periodand second time period generally begin simultaneously. Thus, it shouldbe understood that the portion of the curable sealant adjacent thesurface of the aircraft component is allowed to remain liquid at thesame time and subsequent to forming the non-tacky skin.

Aircraft exterior and interior surfaces, to which sealants may beapplied, may include metals such as titanium, stainless steel, andaluminum, and/or composites, any of which may be anodized, primed,organic-coated or chromate-coated.

In some embodiments, curable compositions useful for practicing thepresent disclosure comprise at least one adhesion promoter. Adhesionpromoter may be present in amount from 0.1 wt % to 15 wt % of thecurable sealant, less than 5 wt %, less than 2 wt %, and in someembodiments, less than 1 wt %, based on the total weight of the curablesealant. Examples of adhesion promoters include phenolics, such as aphenolic resin available under the trade designation “METHYLON”, epoxyresins such as low molecular weight bisphenol A diglycidyl ethers,organosilanes, such as epoxy-, mercapto- or amino-functional silanes,organotinates, and organozirconates. Examples of mercaptosilanes usefulas adhesion promoters include gamma-mercaptopropyltrimethoxysilane,gamma-mercaptopropyltriethoxysilane,gamma-mercaptopropylmethyldimethoxysilane,gamma-mercaptopropylmethyldiethoxysilane,mercaptomethyltrimethoxysilane, mercaptomethyltriethoxysilane, andcombinations thereof. In some embodiments, useful organosilanes haveamino functional groups (e.g.,N-2-(aminoethyl)-3-aminopropyltrimethoxysilane and(3-aminopropyl)trimethoxysilane). In some embodiments, useful adhesionpromoters have groups polymerizable by, for example, actinic radiation.Examples of polymerizable moieties are materials that contain olefinicfunctionality such as styrenic, vinyl (e.g., vinyltriethoxysilane,vinyltri(2-methoxyethoxy) silane), acrylic and methacrylic moieties(e.g., 3-metacrylroxypropyltrimethoxysilane). Some functional silanesuseful as adhesion promoters are commercially available, for example,from Momentive Performance Materials, Inc., Waterford, N.Y., under thetrade designations “SILQUEST A-187” and “SILQUEST A-1100”. Other usefuladhesion promoters are known in the art. In some embodiments ofmercaptan-functional adhesion promoters, the adhesion promoter has amercaptan equivalent weight of less than 5000, 4000, 3000, 2000, or 1000as determined by mercaptan titration so that they may more easilymigrate within the curable sealant composition. Other functionaladhesion promoters (e.g., amino- or epoxy-silanes) can also haveequivalent weights of less than 5000, 4000, 3000, 2000, or 1000 asdetermined by titration. Typical titanate and zirconate coupling agentsare known to those skilled in the art and a detailed overview of theuses and selection criteria for these materials can be found in Monte,S.J., Kenrich Petrochemicals, Inc., “Ken-React® ReferenceManual—Titanate, Zirconate and Aluminate Coupling Agents”, Third RevisedEdition, March, 1995.

Examples of suitable wetting agents include a silicone, modifiedsilicone, silicone acrylate, hydrocarbon solvent, fluorine-containingcompound, non-silicone polymer or copolymer such as a copolyacrylate,and mixtures thereof. Examples of nonionic surfactants suitable aswetting agents in the curable sealants disclosed herein include blockcopolymers of polyethylene glycol and polypropylene glycol,polyoxyethylene (7) lauryl ether, polyoxyethylene (9) lauryl ether,polyoxyethylene (18) lauryl ether, and polyethoxylated alkyl alcoholssuch as those available, for example, from Air Products and ChemicalsInc., Allentown, Penn., under the trade designation “SURFYNOL SE-F”.Fluorochemical surfactants such as those available under the tradedesignation “FLUORAD” from 3M Company of St. Paul, Minn.) may also beuseful. In some embodiments, the curable sealant useful for practicingthe present disclosure includes at least about 0.001 wt %, at leastabout 0.01 wt %, or at least about 0.02 wt % of at least one wettingagent and up to about 2 wt %, up to about 1.5 wt %, or up to about 1 wt% of at least one wetting agent.

In some embodiments of the method according to the present disclosure,the curable sealant comprises a polythiol comprising more than one thiolgroup. In some embodiments, the polythiol includes at least two thiolgroups. Generally, in order to achieve chemical crosslinking betweenpolymer chains, greater than two thiol groups and/or greater than twocrosslinking groups are present in at least some of the polythiol andcuring agent molecules, respectively. In some embodiments, mixtures ofcuring agents and/or polythiols having at least 5 percent functionalequivalents of thiol groups contributed by polythiols having at leastthree thiol groups may be useful.

A variety of polythiols having more than one thiol group are useful inthe method according to the present disclosure. In some embodiments, thepolythiol is monomeric. In these embodiments, the polythiol may be analkylene, arylene, alkylarylene, arylalkylene, or alkylenearylalkylenehaving at least two mercaptan groups, wherein any of the alkylene,alkylarylene, arylalkylene, or alkylenearylalkylene are optionallyinterrupted by one or more ether (i.e., —O—), thioether (i.e., —S—), oramine (i.e., —NR¹—) groups and optionally substituted by alkoxy orhydroxyl. Useful monomeric polythiols may be dithiols or polythiols withmore than 2 (in some embodiments, 3 or 4) mercaptan groups. In someembodiments, the polythiol is an alkylene dithiol in which the alkyleneis optionally interrupted by one or more ether (i.e., —O—) or thioether(i.e., —S—) groups. Examples of useful dithiols include1,2-ethanedithiol, 1,2-propanedithiol, 1,3-propanedithiol,1,3-butanedithiol, 1,4-butanedithiol, 2,3-butanedithiol,1,3-pentanedithiol, 1,5-pentanedithiol, 1,6-hexanedithiol,1,3-dimercapto-3-methylbutane, dipentenedimercaptan,ethylcyclohexyldithiol (ECHDT), dimercaptodiethylsulfide,methyl-substituted dimercaptodiethylsulfide, dimethyl-substituteddimercaptodiethylsulfide, dimercaptodioxaoctane,1,5-dimercapto-3-oxapentane and mixtures thereof. Examples of polythiolshaving more than two mercaptan groups include propane-1,2,3-trithiol;1,2-bis[(2-mercaptoethyl)thio]-3-mercaptopropane;tetrakis(7-mercapto-2,5-dithiaheptyl)methane; and trithiocyanuric acid.Combination of any of these or with any of the dithiols mentioned abovemay be useful.

In some embodiments, the polythiol in the method according to thepresent disclosure is oligomeric or polymeric. Examples of usefuloligomeric or polymeric polythiols include polythioethers and polysulfides. Polythioethers include thioether linkages (i.e., —S—) in theirbackbone structures. Polysulfides include disulfide linkages (i.e.,—S—S—) in their backbone structures.

Polythioethers can be prepared, for example, by reacting dithiols withdienes, diynes, divinyl ethers, diallyl ethers, ene-ynes, orcombinations of these under free-radical conditions. Useful dithiolsinclude any of the dithiols listed above. Examples of suitable divinylethers include divinyl ether, ethylene glycol divinyl ether, butanedioldivinyl ether, hexanediol divinyl ether, diethylene glycol divinylether, triethylene glycol divinyl ether, tetraethylene glycol divinylether, cyclohexanedimethanol divinyl ether, polytetrahydrofuryl divinylether, and combinations of any of these. Useful divinyl ethers offormula CH₂═CH—O—(—R²—O—)_(m)—CH═CH₂, in which m is a number from 0 to10, R² is C₂ to C₆ branched alkylene can be prepared by reacting apolyhydroxy compound with acetylene. Examples of compounds of this typeinclude compounds in which R² is an alkyl-substituted methylene groupsuch as —CH(CH₃)-(e.g., those obtained from BASF, Florham Park, N.J,under the trade designation “PLURIOL”, for which R² is ethylene and m is3.8) or an alkyl-substituted ethylene (e.g., —CH₂CH(CH₃)— such as thoseobtained from International Specialty Products of Wayne, N.J., under thetrade designation “DPE” (e.g., “DPE-2” and “DPE-3”). Examples of othersuitable dienes, diynes, and diallyl ethers include4-vinyl-1-cyclohexene, 1,5-cyclooctadiene, 1,6-heptadiyne,1,7-octadiyne, and diallyl phthalate. Small amounts trifunctionalcompounds (e.g., triallyl-1,3,5-triazine-2,4,6-trione,2,4,6-triallyloxy-1,3,5-triazine) may also be useful in the preparationof oligomers.

Examples of oligomeric or polymeric polythioethers useful for practicingthe present disclosure are described, for example, in U.S. Pat. Nos.4,366,307 (Singh et al.), 4,609,762 (Morris et al.), 5,225,472 (Cameronet al.), 5,912,319 (Zook et al.), 5,959,071 (DeMoss et al.), 6,172,179(Zook et al.), and 6,509,418 (Zook et al.). In some embodiments, thepolythioether is represented by formulaHS—R³-[S-(CH₂)₂—O—[-R⁴—O—]_(m)—(CH₂)₂—S—R³—]_(n)—SH, wherein each R³ andR⁴ is independently a C₂₋₆ alkylene, wherein alkylene may bestraight-chain or branched, C₆₋₈ cycloalkylene, C₆₋₁₀alkylcycloalkylene, —[(CH₂—)_(p)—X—]_(q)—(—CH₂—)_(r), in which at leastone —CH₂— is optionally substituted with a methyl group, X is selectedfrom the group consisting of O, S and —NR⁵—, R⁵ denotes hydrogen ormethyl, m is a number from 0 to 10, n is a number from 1 to 60, p is aninteger from 2 to 6, q is an integer from 1 to 5, and r is an integerfrom 2 to 10. Polythioethers with more than two mercaptan groups mayalso be useful.

In some embodiments, a free-radical initiator is combined with thedithiols with dienes, diynes, divinyl ethers, diallyl ethers, ene-ynes,or combinations of these, and the resulting mixture is heated to providethe polythioethers. Examples of suitable free-radical initiators includeazo compounds (e.g., 2,2′-azobisisobutyronitrile (AIBN),2,2′-azobis(2-methylbutyronitrile), or azo-2-cyanovaleric acid). In someembodiments, the free-radical initiator is an organic peroxide. Examplesof useful organic peroxides include hydroperoxides (e.g., cumene,tert-butyl or tert-amyl hydroperoxide), dialkyl peroxides (e.g.,di-tert-butylperoxide, dicumylperoxide, or cyclohexyl peroxide),peroxyesters (e.g., tert-butyl perbenzoate, tert-butylperoxy-2-ethylhexanoate, tert-butyl peroxy-3,5,5-trimethylhexanoate,tert-butyl monoperoxymaleate, or di-tert-butyl peroxyphthalate),peroxycarbonates (e.g., tert-butylperoxy 2-ethylhexylcarbonate,tert-butylperoxy isopropyl carbonate, or di(4-tert-butylcyclohexyl)peroxydicarbonate), ketone peroxides (e.g., methyl ethyl ketoneperoxide, 1,1-di(tert-butylperoxy)cyclohexane,1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, and cyclohexanoneperoxide), and diacylperoxides (e.g., benzoyl peroxide or laurylperoxide). The organic peroxide may be selected, for example, based onthe temperature desired for use of the organic peroxide andcompatibility with the monomers. Combinations of two or more organicperoxides may also be useful.

The free-radical initiator useful for making a polythioether may also bea photoinitiator. Examples of useful photoinitiators include benzoinethers (e.g., benzoin methyl ether or benzoin butyl ether); acetophenonederivatives (e.g., 2,2-dimethoxy-2-phenylacetophenone or2,2-diethoxyacetophenone); 1-hydroxycyclohexyl phenyl ketone; andacylphosphine oxide derivatives and acylphosphonate derivatives (e.g.,bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide,diphenyl-2,4,6-trimethylbenzoylphosphine oxide,isopropoxyphenyl-2,4,6-trimethylbenzoylphosphine oxide, or dimethylpivaloylphosphonate). Many photoinitiators are available, for example,from BASF under the trade designation “IRGACURE”. The photoinitiator maybe selected, for example, based on the desired wavelength for curing andcompatibility with the monomers. When using a photoinitiator, thepolythioether is typically prepared using an actinic light source (e.g.,at least one of a blue light source or a UV light source).

Polythioethers can also be prepared, for example, by reacting dithiolswith diepoxides, which may be carried out by stirring at roomtemperature, optionally in the presence of a tertiary amine catalyst(e.g., 1,4-diazabicyclo[2.2.2]octane (DABCO)). Useful dithiols includeany of those described above. Useful epoxides can be any of those havingtwo epoxide groups. In some embodiments, the diepoxide is a bisphenoldiglycidyl ether, wherein the bisphenol (i.e., —O—C₆H₅—CH₂-C₆H₅—O—) maybe unsubstituted (e.g., bisphenol F), or either of the phenyl rings orthe methylene group may be substituted by halogen (e.g., fluoro, chloro,bromo, iodo), methyl, trifluoromethyl, or hydroxymethyl. Polythioethersprepared from dithiols and diepoxides have pendent hydroxyl groups andcan have structural repeating units represented by formula—S—R³—S—CH₂—CH(OH)—CH₂—O—C₆H₅—CH₂-C₆H₅—O—CH₂—CH(OH)—CH₂—S—R³—S—, whereinR³ is as defined above, and the bisphenol (i.e., —O—C₆H₅—CH₂-C₆H₅—O—)may be unsubstituted (e.g., bisphenol F), or either of the phenyl ringsor the methylene group may be substituted by halogen (e.g., fluoro,chloro, bromo, iodo), methyl, trifluoromethyl, or hydroxymethyl.Mercaptan terminated polythioethers of this type can also be reactedwith any of the dienes, diynes, divinyl ethers, diallyl ethers, andene-ynes listed above under free radical conditions. Any of thefree-radical initiators and methods described above may be useful forpreparing the polythioethers. In some embodiments, the thermalinitiators described above are used, and the resulting mixture is heatedto provide the polythioethers.

Polysulfides are typically prepared by the condensation of sodiumpolysulfide with bis-(2-chloroethyl) formal, which provides linearpolysulfides having two terminal mercaptan groups. Branched polysulfideshaving three or more mercaptan groups can be prepared usingtrichloropropane in the reaction mixture. Examples of usefulpolysulfides are described, for example, in U.S. Pat. No. 2,466,963(Patrick et al); U.S. Pat. No. 2,789,958 (Fettes et al); U.S. Pat. No.4,165,425(Bertozzi); and U.S. Pat. No. 5,610,243 (Vietti et al.). Polysulfides are commercially available under the trademarks “THIOKOL” and“LP” from Toray Fine Chemicals Co., Ltd., Urayasu, Japan and areexemplified by grades “LP-2”, “LP-2C” (branched), “LP-3”, “LP-33”, and“LP-541”.

Polythioethers and poly sulfides can have a variety of useful molecularweights. In some embodiments, the polythioethers and polysulfides havenumber average molecular weights in a range from 500 grams per mole to20,000 grams per mole, 1,000 grams per mole to 10,000 grams per mole, or2,000 grams per mole to 5,000 grams per mole.

In some embodiments, the curable sealant comprises a polyepoxidecomprising more than one epoxide group. Epoxides are useful, forexample, as curing agents for polythiols. In some embodiments, thepolyepoxide includes at least two epoxide groups. Generally, in order toachieve chemical crosslinking between polymer chains, greater than twothiol groups and/or greater than two epoxide groups are present in atleast some of the polythiol and polyepoxide molecules, respectively.When using a polythiol having two thiol groups, for example, a mixtureof polyepoxides may be useful in which at least one polyepoxide has twoepoxide groups, and at least one polyepoxide has at least three epoxidegroups. Mixtures of polyepoxides and/or polythiols having at least 5percent functional equivalents of epoxide groups contributed bypolyepoxides having at least three epoxide groups or at least 5 percentfunctional equivalents of thiol groups contributed by polythiols havingat least three thiol groups may be useful. A variety of polyepoxideshaving more than one epoxide group are useful in the method according tothe present disclosure. In some embodiments, the polyepoxide ismonomeric. In some embodiments, the polyepoxide is oligomeric orpolymeric (that is, an epoxy resin). A monomeric polyepoxide may be analkylene, arylene, alkylarylene, arylalkylene, or alkylenearylalkylenehaving at least two epoxide groups, wherein any of the alkylene,alkylarylene, arylalkylene, or alkylenearylalkylene are optionallyinterrupted by one or more ether (i.e., —O—), thioether (i.e., —S—), oramine (i.e., —NR¹—) groups and optionally substituted by alkoxy,hydroxyl, or halogen (e.g., fluoro, chloro, bromo, iodo). Usefulmonomeric polyepoxides may be diepoxides or polyepoxides with more than2 (in some embodiments, 3 or 4) epoxide groups. An epoxy resin may beprepared by chain-extending any of such polyepoxides.

Some useful polyepoxides are aromatic. Useful aromatic polyepoxides andepoxy resins typically contain at least one (in some embodiments, atleast 2, in some embodiments, in a range from 1 to 4) aromatic ring(e.g., phenyl group) that is optionally substituted by a halogen (e.g.,fluoro, chloro, bromo, iodo), alkyl having 1 to 4 carbon atoms (e.g.,methyl or ethyl), or hydroxyalkyl having 1 to 4 carbon atoms (e.g.,hydroxymethyl). For polyepoxides and epoxy resin repeating unitscontaining two or more aromatic rings, the rings may be connected, forexample, by a branched or straight-chain alkylene group having 1 to 4carbon atoms that may optionally be substituted by halogen (e.g.,fluoro, chloro, bromo, iodo). In some embodiments, the aromaticpolyepoxide or epoxy resin is a novolac. In these embodiments, thenovolac epoxy may be a phenol novolac, an ortho-, meta-, or para-cresolnovolac, or a combination thereof. In some embodiments, the aromaticpolyepoxide or epoxy resin is a bisphenol diglycidyl ether, wherein thebisphenol (i.e., —O—C₆H₅—CH₂-C₆H₅—O—) may be unsubstituted (e.g.,bisphenol F), or either of the phenyl rings or the methylene group maybe substituted by halogen (e.g., fluoro, chloro, bromo, iodo), methyl,trifluoromethyl, or hydroxymethyl. In some embodiments, the polyepoxideis a novolac epoxy resin (e.g., phenol novolacs, ortho-, meta-, orpara-cresol novolacs or combinations thereof), a bisphenol epoxy resin(e.g., bisphenol A, bisphenol F, halogenated bisphenol epoxies, andcombinations thereof), a resorcinol epoxy resin, and combinations of anyof these. Examples of useful aromatic monomeric polyepoxides include thediglycidyl ethers of bisphenol A and bisphenol F and tetrakisglycidyl-4-phenylolethane and mixtures thereof.

Some useful polyepoxides are non-aromatic. The non-aromatic epoxy caninclude a branched or straight-chain alkylene group having 1 to 20carbon atoms optionally interrupted with at least one —O— and optionallysubstituted by hydroxyl. In some embodiments, the non-aromatic epoxy caninclude a poly(oxyalkylene) group having a plurality (x) of oxyalkylenegroups, OR¹, wherein each R¹ is independently C₂ to C₅ alkylene, in someembodiments, C₂ to C₃ alkylene, x is 2 to about 6, 2 to 5, 2 to 4, or 2to 3. Examples of useful non-aromatic monomeric polyepoxides includeethylene glycol diglycidyl ether, propylene glycol diglycidyl ether,diethylene glycol diglycidyl ether, dipropylene glycol diglycidyl ether,polyethylene glycol diglycidyl ether, polypropylene glycol diglycidylether, glycerol diglycidyl ether, propanediol diglycidyl ether,butanediol diglycidyl ether, and hexanediol diglycidyl ether. Examplesof useful polyepoxides having more than two epoxide groups includeglycerol triglycidyl ether, and polyglycidyl ethers of1,1,1-trimethylolpropane, pentaerythritol, and sorbitol. Other examplesof useful polyepoxides include glycidyl ethers of cycloaliphaticalcohols (e.g., 1,4-cyclohexanedimethanol,bis(4-hydroxycyclohexyl)methane or 2,2-bis(4-hydroxycyclohexyl)propane),cycloaliphatic epoxy resins (e.g., bis(2,3-epoxycyclopentyl) ether,2,3-epoxycyclopentyl glycidyl ether,1,2-bis(2,3-epoxycyclopentyloxy)ethane and 3,4-epoxycyclohexylmethyl3′,4′-epoxycyclohexanecarboxylate), and hydantoin diepoxide. Examples ofpolyepoxides having amine groups include poly(N-glycidyl) compoundsobtainable by dehydrochlorinating the reaction products ofepichlorohydrin with amines containing at least two amine hydrogenatoms. These amines are, for example, aniline, n-butylamine,bis(4-aminophenyl)methane, m-xylylenediamine orbis(4-methylaminophenyl)methane. Examples of polyepoxides havingthioether groups include di-S-glycidyl derivatives of dithiols (e.g.,ethane-1,2-dithiol or bis(4-mercaptomethylphenyl) ether).

In some embodiments of compositions useful in the methods according tothe present disclosure, the polyepoxide is an oligomeric or polymericdiepoxide. In some embodiments, epoxides may be chain extended to haveany desirable epoxy equivalent weight. Chain extending epoxy resins canbe carried out by reacting a monomeric diepoxide, for example, with adiol in the presence of a catalyst to make a linear polymer. In someembodiments, the resulting epoxy resin (e.g., either an aromatic ornon-aromatic epoxy resin) may have an epoxy equivalent weight of atleast 150, 170, 200, or 225 grams per equivalent. In some embodiments,the aromatic epoxy resin may have an epoxy equivalent weight of up to2000, 1500, or 1000 grams per equivalent. In some embodiments, thearomatic epoxy resin may have an epoxy equivalent weight in a range from150 to 2000, 150 to 1000, or 170 to 900 grams per equivalent. Epoxyequivalent weights may be selected, for example, so that the epoxy resinmay be used as a liquid.

Mixtures of polythiols and mixtures of polyepoxides, including any ofthose described above, may also be useful. Typically the amounts of thepolythiol(s) and polyepoxide(s) are selected for the composition so thatthere is a stoichiometric equivalence of mercaptan groups and epoxidegroups.

In some embodiments, the curable sealant comprises at least oneunsaturated compound comprising more than one carbon-carbon double bond,carbon-carbon triple bond, or a combination thereof. These unsaturatedcompounds are useful, for example, as curing agents for polythiols. Insome embodiments, the unsaturated compound includes at least twocarbon-carbon double bonds, carbon-carbon triple bonds, or combinationsthereof. Generally, in order to achieve chemical crosslinking betweenpolymer chains, greater than two thiol groups and/or greater than twocarbon-carbon double bonds, carbon-carbon triple bonds, or acombinations thereof are present in at least some of the polythiol andunsaturated compounds, respectively. It should be understood that theunsaturated compound having carbon-carbon double bonds and/orcarbon-carbon triple bonds are reactive and generally not part of anaromatic ring. In some of these embodiments, the carbon-carbon doubleand triple bonds are terminal groups in a linear aliphatic compound.However, styryl groups and allyl-substituted aromatic rings may beuseful. The unsaturated compound may also include one or more ether(i.e., —O—), thioether (i.e., —S—), amine (i.e., —NR¹—), or ester (e.g.,so that the compound is an acrylate or methacrylate) groups and one ormore alkoxy or hydroxyl substituents. In some embodiments, theunsaturated compound does not include ester groups or carbonate groups.In these embodiments, the unsaturated compound is not an acrylate,methacrylate, vinyl ester, or vinyl carbonate. Unsaturated compoundswithout ester and carbonate groups may be more chemically stable thanunsaturated compounds that contain these groups. Suitable unsaturatedcompounds include dienes, diynes, divinyl ethers, diallyl ethers,ene-ynes, and trifunctional versions of any of these. Combinations ofany of these groups may also be useful. Examples of useful unsaturatedcompounds having more than one carbon-carbon double bond and/orcarbon-carbon triple bond include any of those described above inconnection with the preparation of polythioethers. When using polythiolshaving two thiol groups, a mixture of unsaturated compounds may beuseful in which at least one unsaturated compound has two carbon-carbondouble or triple bonds, and at least one unsaturated compound has atleast three carbon-carbon double or triple bonds. Mixtures ofunsaturated compounds having at least 5 percent functional equivalentsof carbon-carbon double or triple bonds contributed by polyenes havingat least three carbon-carbon double or triple bonds may be useful.

In some embodiments, curable sealants useful for practicing the presentdisclosure include a Michael acceptor comprising more than one Michaelacceptor group. A “Michael acceptor” refers to an activated alkene, suchas an alkenyl group proximate to an electron-withdrawing group such as aketone, nitro, halo, nitrile, carbonyl, or nitro group. Michaelacceptors are well known in the art. A “Michael acceptor group” refersto an activated alkenyl group and an electron-withdrawing group. In someembodiments, a Michael acceptor comprises at least one of a vinylketone, a vinyl sulfone, a quinone, an enamine, a ketimine, oxazolidine,an acrylate, acrylonitrile, acrylamides, maleimides, alkylmethacrylates, cyanoacrylate, alpha, beta-unsaturated aldehydes, vinylphosphonates, vinyl pyridines, beta-keto acetylenes, and acetyleneesters. In some embodiments, the composition is substantially free of aMichael acceptor. “Substantially free” refers to having up to 5, 4, 3,2, or 1 percent by weight of a Michael acceptor, based on the totalweight of the composition. “Substantially free” of a Michael acceptoralso includes being free of a Michael acceptor.

Curable sealants useful for practicing the method of the presentdisclosure can also contain fillers. Conventional inorganic fillers suchas silica (e.g., fumed silica), calcium carbonate, aluminum silicate,and carbon black may be useful as well as low density fillers. In someembodiments, the curable sealant disclosed herein includes at least oneof silica, hollow ceramic elements, hollow polymeric elements, calciumsilicates, calcium carbonate, or carbon black. Silica, for example, canbe of any desired size, including particles having an average size above1 micrometer, between 100 nanometers and 1 micrometer, and below 100nanometers. Silica can include nanosilica and amorphous fumed silica,for example Suitable low density fillers may have a specific gravityranging from about 1.0 to about 2.2 and are exemplified by calciumsilicates, fumed silica, precipitated silica, and polyethylene. Examplesinclude calcium silicate having a specific gravity of from 2.1 to 2.2and a particle size of from 3 to 4 microns (“HUBERSORB HS-600”, J. M.Huber Corp.) and fumed silica having a specific gravity of 1.7 to 1.8with a particle size less than 1 (“CAB—O—SIL TS-720”, Cabot Corp.).Other examples include precipitated silica having a specific gravity offrom 2 to 2.1 (“HI-SIL TS-7000”, PPG Industries), and polyethylenehaving a specific gravity of from 1 to 1.1 and a particle size of from10 to 20 microns (“SHAMROCK S-395” Shamrock Technologies Inc.). The term“ceramic” refers to glasses, crystalline ceramics, glass-ceramics, andcombinations thereof. Hollow ceramic elements can include hollow spheresand spheroids. The hollow ceramic elements and hollow polymeric elementsmay have one of a variety of useful sizes but typically have a maximumdimension of less than 10 millimeters (mm), more typically less than onemm. The specific gravities of the microspheres range from about 0.1 to0.7 and are exemplified by polystyrene foam, microspheres ofpolyacrylates and polyolefins, and silica microspheres having particlesizes ranging from 5 to 100 microns and a specific gravity of 0.25(“ECCOSPHERES”, W. R. Grace & Co.). Other examples include elastomericparticles available, for example, from Akzo Nobel, Amsterdam, TheNetherlands, under the trade designation “EXPANCEL”. Other examplesinclude alumina/silica microspheres having particle sizes in the rangeof 5 to 300 microns and a specific gravity of 0.7 (“FILLITE”,Pluess-Stauffer International), aluminum silicate microspheres having aspecific gravity of from about 0.45 to about 0.7 (“Z-LIGHT”), andcalcium carbonate-coated polyvinylidene copolymer microspheres having aspecific gravity of 0.13 (“DUALITE 6001AE”, Pierce & Stevens Corp.).Further examples of commercially available materials suitable for use ashollow, ceramic elements include glass bubbles marketed by 3M Company,Saint Paul, Minnesota, as “3M GLASS BUBBLES” in grades K1, K15, K20,K25, K37, K46, S15, S22, S32, S35, S38, S38HS, S38XHS, S42HS, S42XHS,S60, S60HS, iM30K, iM16K, XLD3000, XLD6000, and G-65, and any of the HGSseries of “3M GLASS BUBBLES”; glass bubbles marketed by PottersIndustries, Carlstadt, N.J., under the trade designations “Q-CEL HOLLOWSPHERES” (e.g., grades 30, 6014, 6019, 6028, 6036, 6042, 6048, 5019,5023, and 5028); and hollow glass particles marketed by Silbrico Corp.,Hodgkins, Ill. under the trade designation “SIL-CELL” (e.g., grades SIL35/34, SIL-32, SIL-42, and SIL-43). Such fillers, alone or incombination, can be present in a sealant in a range from 10 percent byweight to 55 percent by weight, in some embodiments, 20 percent byweight to 50 percent by weight, based on the total weight of the curablesealant composition. The presence of filler in the curable sealantprovides the advantageous effect of increasing the open time of thecurable sealant in some cases.

Curable sealants useful for practicing the method of the presentdisclosure can also contain at least one of cure accelerators, colorants(e.g., pigments and dyes), thixotropic agents, and solvents. The solventcan conveniently be any material (e.g., N-methyl-2-pyrrolidone,tetrahydrofuran, ethyl acetate, or those described below) capable ofdissolving a component of the curable sealant. Suitable pigments anddyes can include those that do not absorb in the wavelength range thatis desirable for curing the composition. Examples of pigments and dyesuseful in the compositions according to the present disclosure can befound in co-pending U.S. Pat. App. Serial No. 62/416,958 (Townsend etal.), filed on Nov. 3, 2016.

In some embodiments, curable sealants useful for practicing the methodof the present disclosure include at least one oxidizing agent.Oxidizing agents can be useful, for example, when the curable sealantincludes a poly sulfide oligomer or polymer. In some embodiments,oxidizing agents can minimize the degradation or interchanging ofdisulfide bonds in the sealant network. In other embodiments, oxidizingagents can be a component for curing the curable sealant. Usefuloxidizing agents include a variety of organic and inorganic oxidizingagents (e.g., organic peroxides and metal oxides). Examples of metaloxides useful as oxidizing agents include calcium dioxide, manganesedioxide, zinc dioxide, lead dioxide, lithium peroxide, and sodiumperborate hydrate. Other useful inorganic oxidizing agents includesodium dichromate. Examples of organic peroxides useful as oxidizingagents include hydroperoxides (e.g., cumene, tert-butyl or tert-amylhydroperoxide), dialkyl peroxides (e.g., di-tert-butylperoxide,dicumylperoxide, or cyclohexyl peroxide), peroxyesters (e.g., tert-butylperbenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-butylperoxy-3,5,5-trimethylhexanoate, tert-butyl monoperoxymaleate, ordi-tert-butyl peroxyphthalate), peroxycarbonates (e.g., tert-butylperoxy2-ethylhexylcarbonate, tert-butylperoxy isopropyl carbonate, ordi(4-tert-butylcyclohexyl) peroxydicarbonate), ketone peroxides (e.g.,methyl ethyl ketone peroxide, 1,1-di(tert-butylperoxy)cyclohexane,1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, and cyclohexanoneperoxide), and diacylperoxides (e.g., benzoyl peroxide or laurylperoxide). Other useful organic oxidizing agents include para-quinonedioxime.

In some embodiments of the method according to the present disclosure,forming a non-tacky skin on an exposed portion of the curable sealantcomprises exposing the exposed portion of the curable sealant to actinicradiation. In some of these embodiments, the curable sealant includes aphotoinitiator. Photoinitiators suitable for curing a polythiol with acuring agent comprising an unsaturated compound having at least onecarbon-carbon double bond and/or carbon-carbon triple bond include afree-radical photoinitiator. In some embodiments, the free radicalphotoinitiator is a cleavage-type photoinitiator. Cleavage-typephotoinitiators include acetophenones, alpha-aminoalkylphenones, benzoinethers, benzoyl oximes, acylphosphine oxides and bisacylphosphine oxidesand mixtures thereof. Examples of useful photoinitiators include benzoinethers (e.g., benzoin methyl ether or benzoin butyl ether); substitutedacetophenone (e.g., 2,2-dimethoxy-2-phenylacetophenone or2,2-diethoxyacetophenone); 1-hydroxycyclohexyl phenyl ketone; andacylphosphonate derivatives (e.g.,bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide,diphenyl-2,4,6-trimethylbenzoylphosphine oxide,isopropoxyphenyl-2,4,6-trimethylbenzoylphosphine oxide, or dimethylpivaloylphosphonate). Other useful photoinitiators include thosedescribed above in connection with the preparation of polythioethers.

Many photoinitiators are available, for example, from BASF under thetrade designation “IRGACURE”. The photoinitiator may be selected, forexample, based on the desired wavelength for curing and compatibilitywith the curable sealant. Two or more of any of these photoinitiatorsmay also be used together in any combination.

In some embodiments, a photoinitiator can be added to the curablesealant before it is applied to the surface of the aircraft component.For example, the curable sealant can be packaged as a one-part productincluding the photoinitiator, or a two-part product in which at leastone of the parts includes the photoinitiator can be mixed just before itis applied to surface of the aircraft component. The photoinitiator canbe added to the curable sealant in any amount suitable to initiatecuring. In some embodiments, the photoinitiator is present in an amountin a range from 0.05 weight percent to about 5 weight percent (in someembodiments, 0.1 weight percent to 2.5 weight percent, or 0.1 weightpercent to 2 weight percent), based on the total weight of the curablesealant. In some embodiments, a solution of the photoinitiator can beapplied to the exposed surface of the curable sealant after the curablesealant is applied to the surface of the aircraft component. Furtherdetails about applying a solution of a photoinitiator to the exposedportion of the curable sealant before exposing the exposed portion ofthe curable sealant to actinic radiation can be found below inconnection with the discussion of the application of photolatert bases.

In some embodiments of the method according to the present disclosure,curable sealants in which a non-tacky skin is formed on an exposedsurface thereof using a free-radical photoinitiator also include asecond initiator or initiator system. The presence of a second initiatorcan be useful, for example, for curing the remaining uncured sealantafter the second time period has passed. In some embodiments, the secondinitiator comprises s a peroxide and an amine, wherein the peroxide andthe amine together provide a peroxide-amine redox initiator. In someembodiments, the amine is a tertiary amine In some embodiments, theamine is selected from the group consisting ofdihydroxyethyl-p-toluidine, N,N-diisopropylethylamine, and N, N, N′, N″,N″-pentamethyl-diethylenetriamine. In some embodiments, the peroxide isselected from the group consisting of di-tert-butyl peroxide, methylethyl ketone peroxide, and benzoyl peroxide.

In some embodiments, the second initiator comprises an organichydroperoxide either alone or in combination with a nitrogen-containingbase. Organic hydroperoxides have the general structure R—OOH, wherein Ris an alkyl group, aryl group, arylalkylene group, alkylarylene group,alkylarylenealkylene group, or a combination thereof. Examples of usefulorganic hydroperoxides include cumene hydroperoxide, tert-butylhydroperoxide, tert-amyl hydroperoxide, 1,1,3,3-tetramethylbutylhydroperoxide, isopropylcumyl hydroperoxide, p-menthane hydroperoxide(i.e., 1-methyl-1-(4-methylcyclohexypethyl hydroperoxide),diisopropylbenzene hydroperoxide (e.g., 3,5-diisopropylhydroperoxide).In some embodiments, the organic hydroperoxide includes a ketoneperoxide (e.g., methyl ethyl ketone peroxide, acetone peroxide, andcyclohexanone peroxide). While organic hydroperoxides tend to be some ofthe more stable peroxides and require some of the highest temperaturesfor thermal initiation, we have found that in the presence of apolythiol and unsaturated compound in the composition of the presentdisclosure, the organic hydroperoxide can initiate curing at roomtemperature. In some embodiments, compositions according to the presentdisclosure further comprise a nitrogen-containing base. In someembodiments, a combination of a nitrogen-containing base and an organichydroperoxide can be considered a redox initiator. The nitrogen atom(s)in the nitrogen-containing base can be bonded to alkyl groups, arylgroups, arylalkylene groups, alkylarylene, alkylarylenealkylene groups,or a combination thereof. The nitrogen-containing base can also be acyclic compound, which can include one or more rings and can be aromaticor non-aromatic (e.g., saturated or unsaturated). Cyclicnitrogen-containing bases can include a nitrogen as at least one of theatoms in a 5- or 6-membered ring. In some embodiments, thenitrogen-containing base includes only carbon-nitrogen,nitrogen-hydrogen, carbon-carbon, and carbon-hydrogen bonds. In someembodiments, the nitrogen-containing base can be substituted with atleast one of alkoxy, aryl, arylalkylenyl, haloalkyl, haloalkoxy,halogen, nitro, hydroxy, hydroxyalkyl, mercapto, cyano, aryloxy,arylalkyleneoxy, heterocyclyl, or hydroxyalkyleneoxyalkylenyl. In someembodiments, the nitrogen-containing base is a tertiary amine Examplesof useful tertiary amines include triethylamine, dimethylethanolamine,benzyldimethylamine, dimethylaniline, tribenzylamine, triphenylamine,N,N-dimethyl-para-toluidine, N,N-dimethyl-ortho-toluidine,tetramethylguanidine (TMG), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),1,5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,4-diazabicyclo[2.2.2]octane(DABCO), quinuclidine, dimethylaminomethyl phenol,tris(dimethylaminomethyl)phenol, N,N-dihydroxyethyl-p-toluidine,N,N-diisopropylethylamine, and N, N, N′, N″,N″-pentamethyl-diethylenetriamine. Useful nitrogen-containing bases alsoinclude guanidines such as diphenylguanidine (DPG). In some embodiments,the nitrogen-containing base comprises a substituted or unsubstitutednitrogen-containing ring. In some embodiments, the substituted orunsubstituted nitrogen-containing ring has 5 or 6 atoms in the ring. Thesubstituted or unsubstituted nitrogen-containing ring can be aromatic ornonaromatic and can have up to 4 nitrogen atoms in the ring. The ringcan optionally include other heteroatoms (e.g., S and O). Substitutedaromatic or nonaromatic rings can be substituted by one or moresubstituents independently selected from the group consisting of alkyl,aryl, arylalkylenyl, alkoxy, haloalkyl, haloalkoxy, halogen, nitro,hydroxy, hydroxyalkyl, mercapto, cyano, aryloxy, arylalkyleneoxy,heterocyclyl, hydroxyalkyleneoxyalkylenyl, amino, alkylamino,dialkylamino, (dialkylamino)alkyleneoxy, and oxo. The alkyl substituentcan be unsubstituted or substituted by at least one of alkoxy having upto 4 carbon atoms, halo, hydroxy, or nitro. In some embodiments, thearyl or arylalkylenyl is unsubstituted or substituted by at least one ofalkyl having up to 4 carbon atoms, alkoxy having up to 4 carbon atoms,halo, hydroxy, or nitro. In some embodiments, the nitrogen-containingbase is a substituted or unsubstituted pyridine, pyrazine, imidazole,pyrazole, tetrazole, triazole, oxazole, thiazole, pyrimidine,pyridazine, triazine, tetrazine, or pyrrole. Any of these may besubstituted with halogen (e.g., iodo, bromo, chloro, fluoro), alkyl(e.g., having from 1 to 4, 1 to 3, or 1 to 2 carbon atoms),arylalkylenyl (e.g., benzyl), or aryl (phenyl). In some embodiments, thenitrogen-containing base, is a substituted or unsubstituted imidazole orpyrazole. The imidazole or pyrazole may be substituted with halogen(e.g., iodo, bromo, chloro, fluoro), alkyl (e.g., having from 1 to 4, 1to 3, or 1 to 2 carbon atoms), arylalkylenyl (e.g., benzyl), or aryl(phenyl). Examples of useful nitrogen-containing rings include1-benzylimidazole, 1,2-dimethylimidazole, 4-iodopyrazole,1-methylbenzimidazole, 1-methylpyrazole, 3-methylpyrazole,4-phenylimidazole, and pyrazole.

Organic peroxides, in some embodiments, organic hydroperoxides, can beadded in any amount suitable to initiate curing. In some embodiments,the organic peroxide is present in an amount in a range from 0.05 weightpercent to about 10 weight percent (in some embodiments, 0.1 weightpercent to 5 weight percent, or 0.5 weight percent to 5 weight percent).The organic peroxide and its amount may be selected to provide thecomposition with a desirable second time period (that is, the length oftime a portion of the curable sealant adjacent the surface of theaircraft remains liquid) after it is mixed or thawed. In someembodiments, the composition has an open time of at least 10 minutes, atleast 30 minutes, at least one hour, or at least two hours.

The nitrogen-containing base, which in some embodiments, provides aredox curing system in the presence of an organic peroxide, and itsamount may be selected to provide the composition with a desirablesecond time period (that is, the length of time a portion of the curablesealant adjacent the surface of the aircraft remains liquid) after it ismixed or thawed. In some embodiments, the composition has an open timeof at least 10 minutes, at least 30 minutes, at least one hour, or atleast two hours. The amount of the nitrogen-containing base and itsconjugate acid pKa can both affect the open time. A composition with asmaller amount of a nitrogen-containing base having a higher pKa mayhave the same open time as a composition having a larger amount of anitrogen-containing base having a lower pKa. In some embodiments, thenitrogen-containing base is present in an amount in a range from 0.05weight percent to about 10 weight percent (in some embodiments, 0.1weight percent to 5 weight percent, or 0.5 weight percent to 5 weightpercent).

Photoinitiators suitable for curing a polythiol with a curing agentcomprising polyepoxide having more than one epoxide group include aphotolatert base. A photolatert base photochemically generates a basethat can catalyze the reaction between the polythiol and thepolyepoxide. In some embodiments of the method disclosed herein, thebase is a first amine Photolatert bases are also useful, for example,for curing a polythiol with a curing agent comprising a Michaelacceptor.

A variety of photolatert bases can be useful in the method of thepresent disclosure. Many useful photolatert bases, any of which may beuseful for practicing the present disclosure, have been reviewed inSuyama, K. and Shirai, M., “Photobase Generators: Recent Progress andApplication Trend in Polymer Systems” Progress in Polymer Science 34(2009) 194-209. Photolatert bases useful for practicing the presentdisclosure include photocleavable carbamates (e.g., 9-xanthenylmethyl,fluorenylmethyl, 4-methoxyphenacyl, 2,5-dimethylphenacyl, benzyl, andothers), which have been shown to generate primary or secondary aminesafter photochemical cleavage and liberation of carbon dioxide. Otherphotolatert bases described in the review as useful for generatingprimary or secondary amines include certain O-acyloximes, sulfonamides,and formamides. Acetophenones, benzophenones, and acetonaphthonesbearing quaternary ammonium substituents are reported to undergophotocleavage to generate tertiary amines in the presence of a varietyof counter cations (borates, dithiocarbamates, and thiocyanates).Examples of these photolatert ammonium salts areN-(benzophenonemethyl)tri-N-alkyl ammonium triphenylborates. Certainsterically hindered α-aminoketones are also reported to generatetertiary amines

Recently, quaternary ammonium salts made from a variety of amines andphenylglyoxylic acid have been shown to generate amines that catalyze athiol/epoxy reaction after exposure to UV light. (See Salmi, H., et al.“Quaternary Ammonium Salts of Phenylglyoxylic acid as PhotobaseGenerators for Thiol-Promoted Epoxide Photopolymerization” PolymerChemistry 5 (2014) 6577-6583.) Such salts are also suitable asphotolatert bases useful for practicing the present disclosure.

In some embodiments, the photolatert base useful for practicing thepresent disclosure is a 1,3-diamine compound represented by the formulaN(R₇)(R₆)—CH(R₅)—N(R₄)—C(R₁)(R₂)(R₃) such as those described in U.S.Pat. No. 7,538,104 (Baudin et al.). Such compounds can be consideredarylalkylenyl substituted reduced amidines or guanidines In formulaN(R₇)(R₆)—CH(R₅)—N(R₄)—C(R₁)(R₂)(R₃), R₁ is selected from aromaticradicals, heteroaromatic radicals, and combinations thereof that absorblight in the wavelength range from 200 nm to 650 nm and that areunsubstituted or substituted one or more times by at least onemonovalent group selected from alkyl, alkenyl, alkynyl, haloalkyl, —NO₂,—NR₁₀R₁₁, —CN, —OR₁₂, —SR₁₂, —C(O)R₁₃, —C(O)OR₁₄, halogen, groups of theformula N(R₇)(R₆)—CH(R₅)—N(R₄)—C(R₂)(R₃)— where R₂-R₇ are as definedbelow, and combinations thereof, and that upon absorption of light inthe wavelength range from 200 nm to 650 nm bring about aphotoelimination that generates an amidine or guanidine R₂ and R₃ areeach independently selected from hydrogen, alkyl, phenyl, substitutedphenyl (that is, substituted one or more times by at least onemonovalent group selected from alkyl, —CN, —OR₁₂, —SR₁₂, halogen,haloalkyl, and combinations thereof), and combinations thereof; R₅ isselected from alkyl, —NR₈R₉, and combinations thereof; R₄, R₆, R₇, R₈,R₉ R₁₀ and R₁₁ are each independently selected from hydrogen, alkyl, andcombinations thereof; or R₄ and R₆ together form a C₂-C₁₂ alkylenebridge that is unsubstituted or is substituted by one or more monovalentgroups selected from C₁-C₄ alkyl radicals and combinations thereof; orR₅ and R₇, independently of R₄ and R₆, together form a C₂-C₁₂ alkylenebridge that is unsubstituted or is substituted by one or more monovalentgroups selected from C₁-C₄ alkyl radicals and combinations thereof; or,if R₅ is —NR₈R₉, then R₇ and R₉ together form a C₂-C₁₂ alkylene bridgethat is unsubstituted or is substituted by one or more monovalent groupsselected from C₁-C₄ alkyl radicals and combinations thereof; and R₁₂,R₁₃, and R₁₄ are each independently selected from hydrogen, alkyl, andcombinations thereof. Any of the alkyl and haloalkyl groups above can belinear or branched and, in some embodiments, contain 1 to about 19carbon atoms (in some embodiments, 1 to about 18, 1 to about 12, or 1 toabout 6 carbon atoms). In some embodiments, halogen atoms are chlorine,fluorine, and/or bromine (in some embodiments, chlorine and/orfluorine). The alkenyl groups can be linear or branched and, in someembodiments, contain 2 to about 18 carbon atoms (in some embodiments, 2to about 12 or 2 to about 6 carbon atoms). The alkynyl groups can belinear or branched and, in some embodiments, contain 2 to about 18carbon atoms (in some embodiments, 2 to about 12 or 2 to about 6 carbonatoms).

In some embodiments of formula N(R₇)(R₆)—CH(R₅)—N(R₄)—C(R₁)(R₂)(R₃),R_(l) is selected from substituted and unsubstituted phenyl, naphthyl,phenanthryl, anthryl, pyrenyl, 5,6,7,8-tetrahydro-2-naphthyl,5,6,7,8-tetrahydro-1-naphthyl, thienyl, benzo[b]thienyl,naphtho[2,3-b]thienyl, thianthrenyl, anthraquinonyl, dibenzofuryl,chromenyl, xanthenyl, thioxanthyl, phenoxathiinyl, pyrrolyl, imidazolyl,pyrazolyl, pyrazinyl, pyrimidinyl, pyridazinyl, indolizinyl, isoindolyl,indolyl, indazolyl, purinyl, quinolizinyl, isoquinolyl, quinolyl,phthalazinyl, naphthyridinyl, quinoxalinyl, quinazolinyl, cinnolinyl,pteridinyl, carbazolyl, β-carbolinyl, phenanthridinyl, acridinyl,perimidinyl, phenanthrolinyl, phenazinyl, isothiazolyl, phenothiazinyl,isoxazolyl, furazanyl, terphenyl, stilbenyl, fluorenyl, phenoxazinyl,and combinations thereof, any of these being unsubstituted orsubstituted one or more times by C₁-C₁₈ alkyl, C₂-C₁₈ alkenyl, C₂-C₁₈alkynyl, C₁-C₁₈ haloalkyl, —NO₂, —NR₁₀R₁₁, —CN, —OR₁₂, —SR₁₂, —C(O)R₁₃,—C(O)OR₁₄, halogen, a radical of the formulaN(R₇)(R₆)—CH(R₅)—N(R₄)—C(R₂)(R₃)—, or a combination thereof, where R₂-R₇and R₁₀-R₁₄ are as defined above. In some embodiments of formulaN(R₇)(R₆)—CH(R₅)—N(R₄)—C(R₁)(R₂)(R₃), R₁ is a substituted orunsubstituted biphenylyl radical, wherein each phenyl group isindependently substituted with from zero to three (preferably, zero orone) substituents selected from C₁-C₁₈ alkyl, C₂-C₁₈ alkenyl, —OH, —CN,—OR₁₀, —SR₁₀, halogen, radicals of the formulaN(R₇)(R₆)—CH(R₅)—N(R₄)—C(R₂)(R₃)—, and combinations thereof, where R₂-R₇and R₁₀-R₁₄ are as defined above. In some embodiments of formulaN(R₇)(R₆)—CH(R₅)—N(R₄)—C(R₁)(R₂)(R₃), R₁ is selected from phenyl,3-methoxyphenyl, 4-methoxyphenyl, 2,4,6-trimethoxyphenyl,2,4-dimethoxyphenyl, and combinations thereof.

In some embodiments of formula N(R₇)(R₆)—CH(R₅)—N(R₄)—C(R₁)(R₂)(R₃), R₂and R₃ each are independently selected from hydrogen, C₁-C₆ alkyl, andcombinations thereof (in some embodiments, both are hydrogen); R₄ and R₆together form a C₂-C₆ alkylene (in some embodiments, C₃ alkylene) bridgethat is unsubstituted or is substituted by one or more groups selectedfrom C₁-C₄ alkyl radicals and combinations thereof; and/or R₅ and R₇together form a C₂-C₆ alkylene (in some embodiments, C₃ or C₅ alkylene)bridge that is unsubstituted or is substituted by one or more groupsselected from C₁-C₄ alkyl radicals and combinations thereof, or, if R₅is —NR₈ R₉, R₉ and R₇ together form a C₂-C₆ alkylene bridge that isunsubstituted or substituted by one or more groups selected from C₁-C₄alkyl radicals and combinations thereof.

Examples of suitable photolatert bases useful for practicing the presentdisclosure include 5-benzyl-1,5-diazabicyclo[4.3.0]nonane,5-(anthracen-9-yl-methyl)-1,5-diaza[4.3.0]nonane,5-(2′-nitrobenzyl)-1,5-diazabicyclo[4.3.0]nonane,5-(4′-cyanobenzyl)-1,5-diazabicyclo [4.3.0]nonane,5-(3′-cyanobenzyl)-1,5-diazabicyclo[4.3.0]nonane,5-(anthraquinon-2-yl-methyl)-1,5-diaza[4.3.0]nonane,5-(2′-chlorobenzyl)-1,5-diazabicyclo [4.3.0]nonane,5-(4′-methylbenzyl)-1,5-diazabicyclo[4.3.0]nonane,5-(2′,4′,6′-trimethylbenzyl)- 1,5-diazabicyclo [4.3. 0]nonane,5-(4′-ethenylbenzyl)-1,5-diazabicyclo[4.3.0]nonane,5-(3′-trimethylbenzyl)-1,5-diazabicyclo[4.3.0]nonane,5-(2′,3′-dichlorobenzyl)-1,5-diazabicyclo[4.3.0]nonane,5-(naphth-2-yl-methyl-1,5-diazabicyclo[4.3.0]nonane,1,4-bis(1,5-diazabicyclo[4.3.0]nonanylmethyl)benzene,8-benzyl-1,8-diazabicyclo[5.4.0]undecane,8-benzyl-6-methyl-1,8-diazabicyclo[5.4.0]undecane,9-benzyl-1,9-diazabicyclo[6.4.0]dodecane, 10-benzyl-8-methyl-1,10-diazabicyclo[7.4.0]tridecane,11-benzyl-1,11-diazabicyclo[8.4.0]tetradecane,8-(2′-chlorobenzyl)-1,8-diazabicyclo[5.4.0]undecane,8-(2′,6′-dichlorobenzyl)-1,8-diazabicyclo[5.4.0]undecane,4-(diazabicyclo[4.3.0]nonanylmethyl)-1,1′-biphenyl,4,4′-bis(diazabicyclo[4.3.0]nonanylmethyl)-11′-biphenyl,5-benzyl-2-methyl-1,5-diazabicyclo[4.3.0]nonane,5-benzyl-7-methyl-1,5,7-triazabicyclo[4.4.0]decane, and combinationsthereof. Such compounds can be made, for example, using the methodsdescribed in U.S. Pat. No. 7,538,104 (Baudin et al.), assigned to BASF,Ludwigshafen, Germany. An example of a photolatert base is availablefrom BASF under the trade designation “CGI 90”, which is reported togenerate 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) upon exposure to actinicradiation (see, e.g., US2013/0345389 (Cai et al.).

Other suitable photolatert bases useful for practicing the presentdisclosure and/or for practicing the methods disclosed herein includethose described in U.S. Pat. No. 6,410,628 (Hall-Goulle et al.), U.S.Pat. No. 6,087,070 (Turner et al.), U.S. Pat. No. 6,124,371 (Stanssenset al.), and U.S. Pat. No. 6,057,380 (Birbaum et al.), and U.S. Pat.Appl. Pub. No. 2011/01900412 (Studer et al.).

In some embodiments, a photolatert base can be added to the curablesealant before it is applied to the surface of the aircraft component.For example, the curable sealant can be packaged as a one-part productincluding the photolatert base, or a two-part product in which at leastone of the parts includes the photolatert base can be mixed just beforeit is applied to surface of the aircraft component. The photolatert basecan be added to the curable sealant in any amount suitable to initiatecuring. In some embodiments, the photolatert base is present in anamount in a range from 0.05 weight percent to about 5 weight percent (insome embodiments, 0.1 weight percent to 2.5 weight percent, or 0.1weight percent to 2 weight percent), based on the total weight of thecurable sealant. In some embodiments, a solution of the photolatert basecan be applied to the exposed surface of the curable sealant after thecurable sealant is applied to the surface of the aircraft component.

In some embodiments of the method according to present disclosure, thecurable sealant is applied to a surface of the aircraft component, andthe non-tacky skin is formed on the exposed portion of the curablesealant upon exposure to actinic radiation. When it is applied, thecurable sealant can comprise a polythiol comprising more than one thiolgroup and a curing agent comprising more than one crosslinkable group.The curable sealant may be stored as a one-part product (e.g., frozen ifnecessary) or stored as a two-part product and mixed shortly before use.A solution comprising a photoinitiator or photolatert base describedabove can then be applied to the surface of the composition. Thesolution comprising the photoinitator or photolatert base can be appliedby any convenient method, for example, dip coating, knife coating,reverse roll coating, brushing, and spraying (e.g., aerosol spraying orelectrostatic spraying). The solution may be allowed to penetrate intothe curable sealant for any desired length of time to allow thephotolatert base to combine with the polythiol and curing agent, forexample In some embodiments, the solution further comprises aphotosensitizer. Following the application of the solution comprisingthe photoinitiator or photolatert base to the surface of thecomposition, a non-tacky skin can be made on the surface by exposing theapplied photoinitiator or photolatert base to an appropriate lightsource. The length of time that the solution is allowed to penetrate thecurable sealant can influence the depth of the light cure and thicknessof the cured skin at the surface of the curable sealant.

The solution including the photoinitiator or photolatert base andoptionally the photosensitizer can include any suitable solvent orsolvents capable of dissolving these components. The components may bepresent in the solvent at any suitable concentration, (e.g., from about5 percent to about 90 percent by weight based on the total weight of thesolution). In some embodiments, each component may be present in a rangefrom 10 to 85 or 25 to 75 percent by weight, based on the total weightof the solution. Illustrative examples of suitable solvents includealiphatic and alicyclic hydrocarbons (e.g., hexane, heptane, andcyclohexane), aromatic solvents (e.g., benzene, toluene, and xylene),ethers (e.g., diethyl ether, glyme, diglyme, and diisopropyl ether),esters (e.g., ethyl acetate and butyl acetate), alcohols (e.g., ethanoland isopropyl alcohol), ketones (e.g., acetone, methyl ethyl ketone, andmethyl isobutyl ketone), sulfoxides (e.g., dimethyl sulfoxide), amides(e.g., N,N-dimethylformamide and N,N-dimethylacetamide), halogenatedsolvents (e.g., methylchloroform, 1,1,2-trichloro-1,2,2-trifluoroethane,trichloroethylene, and trifluorotoluene), and mixtures thereof. When anaromatic photosensitizer is present, an aromatic solvent may be useful.

As shown in the Examples, below, the method according to the presentdisclosure can provide at least a non-tacky skin on the surface of thecomposition even when the composition contains filler. When the sampleswere exposed to 455 nm blue light, cure depths of up to 0.25 millimeterwere achieved. Such cure depths were achieved even when manganesedioxide was used as an oxidant. In polysulfide-based sealants, manganesedioxide is commonly added as an oxidation agent with excess to preventdisulfide bond degradation or interchanging. However, manganese dioxideis black and typically tends to limit the depth of curing.

Applying a solution including a photolatert base to the exposed portionof a curable sealant can be useful, for example, for adding a secondcuring mechanism to an existing product. For example, a traditionalone-part or two-part sealant can include a polythiol, a polyepoxide, andan amine or other accelerator. The accelerator may be present to providethe sealant composition with a balance of a desirable open time and curetime. For example, the composition may be designed to have at least oneof a non-tacky surface or a 30 Shore “A” hardness in less than 24 hours,in some embodiments, less than 12 hours or 10 hours under ambientconditions. The compositions may be designed achieve a 45 to 50 Shore“A” hardness in up to 2 weeks, up to 1 week, up to 5 days, up to 3 days,or up to 1 day. The solution described herein can be sprayed on theexposed surface of the sealant composition, which can then be exposed tolight to provide at least a protective, non-tacky skin on the surface ofthe composition. Underneath the protective skin, the composition cancontinue to cure by means of its accelerator.

In some embodiments of the method according to the present disclosure,curable sealants in which a non-tacky skin is formed on an exposedsurface thereof using a photolatert base also include a second amine Thepresence of a second amine can be useful, for example, for curing theremaining uncured sealant after the second time period has passed. Thesecond amine can also be useful, for example, for curing areas shieldedfrom the light source. The second amine may be the same or differentfrom the first amine In some embodiments, a temperature sufficient forthe second amine to at least partially cure the curable sealant isambient temperature (that is, no external heat source is necessary).

The first amine (generated by the photolatert base) and second amine canindependently be any compound including one to four basic nitrogen atomsthat bear a lone pair of electrons. The first amine and second amine canindependently include primary, secondary, and tertiary amine groups. Thenitrogen atom(s) in the first amine and second amine can be bonded toalkyl groups, aryl groups, arylalkylene groups, alkylarylene,alkylarylenealkylene groups, or a combination thereof. The first amineand second amine can also be cyclic amines, which can include one ormore rings and can be aromatic or non-aromatic (e.g., saturated orunsaturated). One or more of the nitrogen atoms in the amine can be partof a carbon-nitrogen double bond. While in some embodiments, the firstamine and second amine independently include only carbon-nitrogen,nitrogen-hydrogen, carbon-carbon, and carbon-hydrogen bonds, in otherembodiments, the first amine and second amine can include otherfunctional groups (e.g., hydroxyl or ether group). However, it isunderstood by a person skilled in the art that a compound including anitrogen atom bonded to a carbonyl group is an amide, not an amine, andhas different chemical properties from an amine. The first amine andsecond amine can include carbon atoms that are bonded to more than onenitrogen atom. Thus, the first amine and second amine can independentlybe a guanidine or amidine As would be understood by a person skilled inthe art, a lone pair of electrons on one or more nitrogens of the firstamine and second amine distinguishes them from quaternary ammoniumcompounds, which have a permanent positive charge regardless of pH.

Examples of useful first and second amines include propylamine,butylamine, pentylamine, hexylamine, triethylamine,dimethylethanolamine, benzyldimethylamine, dimethylaniline,tribenzylamine, triphenylamine, tetramethylguanidine (TMG),1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),1,5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,4-diazabicyclo[2.2.2]octane(DABCO), quinuclidine, diphenylguanidine (DPG), dimethylaminomethylphenol, and tris(dimethylaminomethyl)phenol. In some embodiments, thefirst amine and second amine are each independently tertiary amines,amidines, or guanidines.

The second amine and its amount may be selected to provide the curablesealant with a desirable amount of open time (that is, the length oftime it takes for the curable sealent to become at least partiallygelled) after it is mixed or thawed and a desirable second time periodduring which it remains liquid at the interface of the aircraftcomponent. In some embodiments, the composition has an open time of atleast 10 minutes, at least 30 minutes, at least one hour, or at leasttwo hours. The amount of the second amine and its conjugate acid pKaboth affect the open time. A composition with a smaller amount of asecond amine having a higher pKa may have the same open time as acomposition having a larger amount of a second amine having a lower pKa.For a second amine with a moderate conjugate acid pKa value in a rangefrom about 7 to about 10, an amount of second amine in a range from 0.05weight percent to about 10 weight percent (in some embodiments, 0.05weight percent to 7.5 weight percent, or 1 weight percent to 5 weightpercent) may be useful. For a second amine with a higher conjugate acidpKa value of about 11 or more, an amount of second amine in a range from0.005 weight percent to about 3 weight percent (in some embodiments,0.05 weight percent to about 2 weight percent) may be useful. In someembodiments in which the second amine is different from the first amine,the second amine has a lower conjugate acid pKa value than the firstamine This may be useful, for example, for achieving a desirable amountof open time and a desirably fast formation of a non-tacky skin. In someembodiments in which the second amine is different from the first amine,the first amine and the second amine have the same conjugate acid pKavalue.

In some embodiments, the second amine may be phase-separated from thecurable sealant. In these embodiments, the second amine can be a solid(e.g., dicyandiamide), present in a solid adduct (e.g., such as anadduct of an amine and an epoxy resin), or segregated within a solid(e.g., a semi-crystalline polymer). As a phase-separated amine, thesecond amine is not reactive with or reacts very slowly with the curablecomponents in the sealant at ambient temperature. Further details aboutcompositions including a phase-separated amine can be found inco-pending U.S. Pat. App. Serial No. 62/416,970 (Zook et al.), filed onNov. 3, 2016. The curable sealant may also include a second amine thatis not phase separated, such as any of those described above, and anamine this phase-separated.

While the first amine is photochemically generated from a photolatertbase, the first and second amines themselves are generally notconsidered photolatert bases. That is, they do not undergo photochemicalreactions that generate an amine by photocleavage, photoelimination, oranother mechanism.

In some embodiments of the method according to present disclosure, thenon-tacky skin is formed on the exposed portion of the curable sealantupon exposure to actinic radiation. In some of these embodiments, usefulphotoinitiators and photolatert bases absorb light in a wavelength rangefrom 200 nm to 650 nm. For some applications, curable sealants thatinclude a photoinitiator or photolatert base absorb light in theultraviolet A (UVA) and/or blue light regions, for example, in awavelength range from 315 nm to 550 nm or 315 nm to 500 nm. UVA lightcan be considered to have a wavelength range of 315 nm to 400 nm, andblue light can be considered to have a wavelength range of 450 nm to 495nm. In some embodiments in which the non-tacky skin is formed uponexposure to actinic radiation, the curable sealant or the solutionincluding a photoinitiator or photolatert base that is applied to thecurable sealant further includes at least one photosensitizer. Aphotosensitizer can be useful, for example, if the photoinitiator orphotolatert base does not have a strong absorbance in a wavelength rangethat is desired for curing the curable sealant. As used herein, aphotosensitizer may be understood to be, for example, a compound havingan absorption spectrum that overlaps or closely matches the emissionspectrum of the radiation source to be used and that can improve theoverall quantum yield by means of, for example, energy transfer orelectron transfer to other component(s) of the curable sealant orsolution (e.g., the photoinitiator or photolatert base). Usefulphotosensitizers include aromatic ketones (e.g., substituted orunsubstituted benzophenones, substituted or unsubstituted thioxanthones,substituted or unsubstituted anthraquinones, and combinations thereof),dyes (e.g., oxazins, acridines, phenazines, rhodamines, and combinationsthereof), 3-acylcoumarins (e.g., substituted and unsubstituted3-benzoylcoumarins and substituted and unsubstituted3-naphthoylcoumarins, and combinations thereof), anthracenes (e.g.,substituted and unsubstituted anthracenes),3-(2-benzothiazolyl)-7-(diethylamino)coumarin (coumarin 6),10-acetyl-2,3,6,7-tetrahydro-1H,5H,11H-[1]benzopyrano[6,7,8-ij]quinolizin-11-one(coumarin 521), other carbonyl compounds (e.g., camphorquinone,4-phenylacetophenone, benzil, and xanthone, and combinations thereof),and combinations thereof. In some embodiments, the photosensitizer hasan absorbance in the blue light range. In some embodiments, thephotosensitizer is camphorquinone. In some embodiments, coumarinphotosensitizers that are triplet photosensitizers with a wavelength ofmaximum absorbance, λ_(max), between 390 to 510 nm are used incombination with camphorquinone. Examples of such coumarinphotosensitizers include 3,3′-carbonylbis(5,7-dimethoxycoumarin),3-benzoyl-7-diethylaminocoumarin, 7-diethylamino-3-thenoylcoumarin,3-(2-benzofuroyl)-7-diethylaminocoumarin,7-diethylamino-5′,7′-dimethoxy-3,3′-carbonylbiscoumarin,3,3′-carbonylbis(7-diethylaminocoumarin),9-(7-diethylamino-3-coumarinoyl)-1,2,4,5-tetrahydro-3H,6H,10H[1]benzopyrano[9,9a,1-gh]quinolazine-10-one,and9,9′-carbonylbis(1,2,4,5-tetrahydro-3H,6H,10H[1]benzopyrano[9,9a,1-gh]quinolazine-10-one). Further details about compositions including a photolatert base,camphorquinone, and such coumarins can be found in co-pending U.S. Pat.App. Ser. No. 62/417,158 (Clough et al.), filed on Nov. 3, 2016. Theamount of photosensitizer can vary widely, depending upon, for example,its nature, the nature of other component(s) of the photoactivatablecomposition, and the particular curing conditions. In embodiments inwhich the photosensitizer is in the solution including thephotoinitiator or photolatert base, the photosensitizer may be presentin the solution at any suitable concentration, (e.g., from about 5percent to about 90 percent by weight, 10 percent to 85 percent byweight, or 25 percent to 75 percent by weight, based on the total weightof the solution). When the photosensitizer is present in the curablesealant, amounts ranging from about 0.1 weight percent to about 15weight percent can be useful. In some embodiments, the photosensitizeris included in the curable sealant in an amount from 0.5 percent to 10percent by weight, 0.5 percent to 7.5 percent by weight, or 1 percent to7.5 percent by weight, based on the total weight of the curable sealant.

The method of making a polymer network according to the presentdisclosure includes exposing the composition disclosed herein in any ofits embodiments to light to generate the first amine to at leastpartially cure at least the surface of the composition. The light sourceand exposure time can be selected, for example, based on the nature andamount of the composition. Sources of ultraviolet and/or visible lightcan be useful (for example, wavelengths ranging from about 200 nm toabout 650 nm, from about 315 nm to 550 nm, or from about 315 nm to 500nm can be useful). Suitable light includes sunlight and light fromartificial sources, including both point sources and flat radiators. Insome embodiments, the light source is a source of at least one of UVA orblue light. In some embodiments, the light source is a blue lightsource.

Examples of useful light sources include carbon arc lamps; xenon arclamps; medium-pressure, high-pressure, and low-pressure mercury lamps,doped if desired with metal halides (metal halogen lamps);microwave-stimulated metal vapor lamps; excimer lamps; superactinicfluorescent tubes; fluorescent lamps; incandescent argon lamps;electronic flashlights; xenon flashlights; photographic flood lamps;light-emitting diodes; laser light sources (for example, excimerlasers); and combinations thereof The distance between the light sourceand the coated substrate can vary widely, depending upon the particularapplication and the type and/or power of the light source. For example,distances up to about 150 cm, distances from about 0.01 cm to 150 cm, ora distance as close as possible without touching the composition can beuseful.

As shown in the Examples, below, the power of the light source can beadjusted to form a non-tacky skin while leaving a portion of curablesealant adjacent the surface of the aircraft component to remain liquidfor a desirable second time period. See, for example, the data in Tables8 and 9. The power level can be selected to form a desirable thicknessof non-tacky skin on the surface of a sealant.

In some embodiments, the curable sealant comprises at least one of anoxygen-activated curing agent or a moisture-activated curing agent. Insome embodiments, the curable sealant comprises an oxygen-activatedcuring agent. For example, polythiols as described above in any of theirembodiments can be combined with a thiuram disulfide in conjunction witha member selected from the group consisting of an iron salt, iron oxide,iron hydroxide, iron metal complex, manganese salt, manganous oxide,manganese hydroxide, and manganese metal complex. For example,polythiols as described above in any of their embodiments can becombined with a dithiocarbamate selected from the group consisting ofiron dithiocarbamate and manganese dithiocarbamate. A non-tacky skin canbe formed on such compositions by exposure to an environment containingoxygen, and a portion of the curable sealant not exposed to oxygen(e.g., adjacent the surface of the aircraft component) will take longerto cure. Further information regarding such oxygen-activated curingagents can be found, for example, in U.S. Pat. No. 3,991,039 (Gunter).

In some embodiments, the curable sealant comprises a moisture-activatedcuring agent. For example, polysulfides as described above in any oftheir embodiments can be combined with an oxidizing agent such asdinitrobenzene, alkali metal peroxides (e.g., sodium peroxide), alkalimetal salt peroxides (e.g., sodium pyrophosphate peroxide, sodiumcarbonate peroxide, sodium perborate), alkaline earth metal peroxides(e.g., calcium peroxide and barium peroxide) and other metal peroxides(e.g., zinc peroxide manganese dioxide), and ammonium dichromate, and analkaline desiccating deliquescent accelerating agent adapted andsufficient to maintain said polymer in dry condition during shipment andstorage and to attract and absorb moisture from its surroundings afterdeposition in place to hasten the curing of said polymer by said curingagent. Such alkaline desiccating deliquescent accelerating agentsinclude sodium oxide, sodium peroxide, potassium hydroxide, sodiumhydroxide, sodium acetate, sodium carbonate, sodium phosphate, sodiummolybdate, calcium oxide, barium oxide, calcium peroxide, bariumperoxide, calcium hydroxide, and strontium hydroxide. A non-tacky skincan be formed on such compositions by exposure to an environmentcontaining moisture, and a portion of the curable sealant not exposed tomoisture (e.g., adjacent the surface of the aircraft component) willtake longer to cure. Further information regarding suchmoisture-activated curing agents can be found, for example, in U.S. Pat.No. 3,225,017 (Seegman). Any of the oxidizing agents described above canalso be used in combination with molecular sieves and a cure acceleratorselected from monomeric and polymeric acrylated liquid polysulfidecompounds having an acrylate functionality of at least 2 to cure amercaptan-terminated liquid polysulfide. A non-tacky skin can be formedon such compositions by exposure to an environment containing moisture.Further details regarding such moisture-activated curing agents can befound, for example, in U.S. Pat. No. 5,409,985 (Robinson).

Curable sealants in the method according to the present disclosure canbe cured into, for example, aviation fuel resistant sealants. Aviationfuel resistant sealants are widely used by the aircraft industry formany purposes. Commercial and military aircraft are typically built byconnecting a number of structural members, such as longitudinalstringers and circular frames. The aircraft skin, whether metal orcomposite, is attached to the outside of the stringers using a varietyof fasteners and adhesives. These structures often include gaps alongthe seams, joints between the rigidly interconnected components, andoverlapping portions of the exterior aircraft skin. The method accordingto the present disclosure can be useful, for example, for sealing suchseams, joints, and overlapping portions of the aircraft skin. Thecurable sealant may be applied, for example, to aircraft fasteners,windows, access panels, and fuselage protrusions. The sealant disclosedherein may prevent the ingress of weather and may provide a smoothtransition between the outer surfaces to achieve desired aerodynamicproperties. The method according to the present disclosure may likewisebe carried out on interior assembles to prevent corrosion, to containthe various fluids and fuels necessary to the operation of an aircraft,and to allow the interior of the aircraft (e.g., the passenger cabin) tomaintain pressurization at higher altitudes. Among these uses are thesealing of integral fuel tanks and cavities.

Aircraft exterior and interior surfaces, to which sealants may beapplied, may include metals such as titanium, stainless steel, andaluminum, and/or composites, any of which may be anodized, primed,organic-coated or chromate-coated. For example, a dilute solution of oneor more phenolic resins, organo-functional silanes, titanates orzirconantes, and a surfactant or wetting agent dissolved in organicsolvent or water may be applied to an exterior or interior surface anddried.

Sealants may optionally be used in combination with a seal cap, forexample, over rivets, bolts, or other types of fasteners. A seal cap maybe made using a seal cap mold, filled with a curable sealant, and placedover a fastener. The curable sealant may then be cured. In someembodiments, the seal cap and the curable sealant may be made from thesame material. For more details regarding seal caps, see, for example,Int. Pat. App. Pub. No. WO2014/172305 (Zook et al.).

In some embodiments, cured sealants prepared from the method accordingto the present disclosure may be useful in these applications, forexample, because of their fuel resistance and low glass transitiontemperatures. In some embodiments, the cured sealant prepared accordingto the present disclosure has a low glass transition temperature, insome embodiments less than −20° C., in some embodiments less than −30°C., in some embodiments less than −40° C., and in some embodiments lessthan −50° C. In some embodiment, the cured sealant prepared according tothe present disclosure has high jet fuel resistance, characterized by avolume swell of less than 30% and a weight gain of less than 20% whenmeasured according to Society of Automotive Engineers (SAE)International Standard AS5127/1.

Some Embodiments of the Disclosure

In a first embodiment, the present disclosure provides a method ofapplying a sealant to an aircraft component, the method comprising:

applying a curable sealant to a surface of the aircraft component,wherein the curable sealant comprises at least one of an adhesionpromoter or a wetting agent; and

forming a non-tacky skin on an exposed portion of the curable sealantwithin a first time periodwhile allowing a portion of the curablesealant adjacent the surface of the aircraft component to be liquid fora second time period, wherein the second time period is at least twicethe first time period.

In a second embodiment, the present disclosure provides the method ofthe first embodiment, wherein the first time period is up to four hours.

In a third embodiment, the present disclosure provides the method of thefirst or second embodiment, wherein the first time period is up to oneminute.

In a fourth embodiment, the present disclosure provides the method ofany one of the first to third embodiments, wherein the non-tacky skin isformed on the exposed portion of the curable sealant within up to onehour, and wherein the portion of the curable sealant adjacent thesurface of the aircraft component remains liquid for at least two hours.

In a fifth embodiment, the present disclosure provides the method of anyone of the first to fourth embodiments, wherein the second time periodis at least ten times the first time period.

In a sixth embodiment, the present disclosure provides the method of anyone of the first to fifth embodiments, wherein the second time period isat least one hundred times the first time period.

In a seventh embodiment, the present disclosure provides the method ofany one of the first to sixth embodiments, wherein the second timeperiod is sufficient to allow for at least one of the adhesion promoteror wetting agent to migrate to the surface of the aircraft component.

In an eighth embodiment, the present disclosure provides the method ofthe any one of the first to seventh embodiments, wherein the sealantdevelops a 30 Shore “A” hardness in less than or equal to 24 hours.

In a ninth embodiment, the present disclosure provides the method of anyone of the first to eighth embodiments, wherein the curable sealant isapplied to a seam or joint between portions of aircraft skin.

In a tenth embodiment, the present disclosure provides the method of anyone of the first to ninth embodiments, wherein the curable sealant isapplied to at least one of an aircraft fastener, an aircraft window, anaircraft access panel, a fuselage protrusion, or an aircraft fuel tank.

In an eleventh embodiment, the present disclosure provide the method ofany one of the first to tenth embodiments, wherein the adhesion promotercomprises at least one of a phenolic resin or an amino-, mercapto-, orepoxy-functional silane.

In a twelfth embodiment, the present disclosure provides the method ofany one of the first to eleventh embodiments, wherein the wetting agentcomprises at least one of a silicone, fluorinated, or hydrocarbonsurfactant.

In a thirteenth embodiment, the present disclosure provides the methodof any one of the first to twelfth embodiments, wherein the curablesealant comprises a polythiol comprising more than one thiol group.

In a fourteenth embodiment, the present disclosure provides the methodof the thirteenth embodiment, wherein the polythiol is monomeric.

In a fifteenth embodiment, the present disclosure provides the method ofthe thirteenth embodiment, wherein the polythiol is oligomeric orpolymeric.

In a sixteenth embodiment, the present disclosure provides the method ofthe fifteenth embodiment, wherein the polythiol is a polythioether.

In a seventeenth embodiment, the present disclosure provides the methodof the sixteenth embodiment, wherein the polythiol is an oligomer orpolymer prepared from components comprising a dithiol and a diene ordivinyl ether.

In an eighteenth embodiment, the present disclosure provides the methodof the fifteenth embodiment, wherein the polythiol is a poly sulfideoligomer or polymer.

In a nineteenth embodiment, the present disclosure provides the methodof the eighteenth embodiment, wherein the curable sealant furthercomprises an oxidizing agent.

In a twentieth embodiment, the present disclosure provides the method ofany one of thirteenth to nineteenth embodiments, wherein the curablesealant comprises a polyepoxide comprising more than one epoxide group.

In a twenty-first embodiment, the present disclosure provides the methodof the twentieth embodiment, wherein the polyepoxide is monomeric.

In a twenty-second embodiment, the present disclosure provides themethod of the twentieth embodiment, wherein the polyepoxide isoligomeric or polymeric.

In a twenty-third embodiment, the present disclosure provides the methodof any one of the twentieth to twenty-second embodiments, wherein thepolyepoxide is aromatic.

In a twenty-fourth embodiment, the present disclosure provides themethod of any one of the twentieth to twenty-second embodiments, whereinthe polyepoxide is non-aromatic.

In a twenty-fifth embodiment, the present disclosure provides the methodof any one of the twentieth to twenty-fourth embodiments, wherein thepolyepoxide comprises three or more epoxide groups.

In a twenty-sixth embodiment, the present disclosure provides the methodof any one of the thirteenth to twenty-fifth embodiments, wherein thecurable sealant comprises a Michael acceptor comprising more than oneMichael acceptor group.

In a twenty-seventh embodiment, the present disclosure provides themethod of any one of the thirteenth to twenty-sixth embodiments, whereinthe curable sealant further comprises a photolatert base catalyst.

In a twenty-eighth embodiment, the present disclosure provides themethod of the twenty-seventh embodiment, wherein forming a non-tackyskin on an exposed portion of the curable sealant comprises exposing theexposed portion of the curable sealant to actinic radiation.

In a twenty-ninth embodiment, the present disclosure provides the methodof any one of the thirteenth to twenty-sixth embodiments, furthercomprising applying a solution comprising a photolatert base catalyst tothe exposed portion of the curable sealant before exposing the exposedportion of the curable sealant to actinic radiation.

In a thirtieth embodiment, the present disclosure provides the method ofthe twenty-ninth embodiment, wherein the solution further comprises asolvent comprising at least one of an aliphatic or alicyclichydrocarbon, an aromatic solvent, ether, ester, alcohol, ketone,sulfoxide, amide, or halogenated solvent.

In a thirty-first embodiment, the present disclosure provides the methodof any one of the twenty-seventh to thirtieth embodiments, wherein thephotolatert base catalyst generates a first amine upon exposure toactinic radiation.

In a thirty-second embodiment, the present disclosure provides themethod of the thirty-first embodiment, wherein the first amine comprisesat least one of a tertiary amine, an amidine, or a guanidine

In a thirty-third embodiment, the present disclosure provides the methodof the thirty-first or thirty-second embodiment, wherein the compositionfurther comprises a catalytic amount of a second amine, which may be thesame or different from the first amine

In a thirty-fourth embodiment, the present disclosure provides themethod of the thirty-third embodiment, wherein at least one of the firstamine or second amine is triethylamine, dimethylethanolamine,benzyldimethylamine, dimethylaniline, tribenzylamine, triphenylamine,tetramethylguanidine (TMG), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),1,5-diazabicyclo[4.3.0]non-5-ene (DBN), 1,4-diazabicyclo[2.2.2]octane(DABCO), quinuclidine, diphenylguanidine (DPG), dimethylaminomethylphenol, and tris(dimethylaminomethyl)phenol.

In a thirty-fifth embodiment, the present disclosure provides the methodof any one of the thirteenth to nineteenth embodiments, wherein thecurable sealant comprises at least one unsaturated compound comprisingmore than one carbon-carbon double bond, carbon-carbon triple bond, or acombination thereof.

In a thirty-sixth embodiment, the present disclosure provides the methodof the thirty-fifth embodiment, wherein the at least one unsaturatedcompound comprises two carbon-carbon double bonds, and wherein thecurable composition further comprises a second unsaturated compoundcomprising three carbon-carbon double bonds.

In a thirty-seventh embodiment, the present disclosure provides themethod of the thirty-fifth or thirty-sixth embodiment, wherein thecurable sealant further comprises a free-radical photoinitiator.

In a thirty-eighth embodiment, the present disclosure provides themethod of the thirty-seventh embodiment, wherein forming a non-tackyskin on an exposed portion of the curable sealant comprises exposing theexposed portion of the curable sealant to actinic radiation.

In a thirty-ninth embodiment, the present disclosure provides the methodof the thirty-fifth or thirty-sixth embodiment, further comprisingapplying a solution comprising a free-radical photoinitiator to theexposed portion of the curable sealant before exposing the exposedportion of the curable sealant to actinic radiation.

In a fortieth embodiment, the present disclosure provides the method ofthe thirty-ninth embodiment, wherein the solution further comprises asolvent comprising at least one of an aliphatic or alicyclichydrocarbon, an aromatic solvent, ether, ester, alcohol, ketone,sulfoxide, amide, or halogenated solvent.

In a forty-first embodiment, the present disclosure provides the methodof any one of the thirty-fifth to fortieth embodiments, wherein thecurable sealant further comprises at least one of a peroxide orhydroperoxide.

In a forty-second embodiment, the present disclosure provides the methodof any one of the thirteenth to nineteenth embodiments, wherein thecurable sealant comprises at least one of an oxygen-activated curingagent or a moisture-activated curing agent.

In a forty-third embodiment, the present disclosure provides an aircraftcomponent sealed by the method of any one of the first to forty-secondembodiments.

In a forty-fourth embodiment, the present disclosure provides anaircraft comprising the aircraft component of the forty-thirdembodiment.

In order that this disclosure can be more fully understood, thefollowing examples are set forth. It should be understood that theseexamples are for illustrative purposes only, and are not to be construedas limiting this disclosure in any manner

EXAMPLES

Unless otherwise noted, all reagents were obtained or are available fromSigma-Aldrich Company, St. Louis, Mo., or may be synthesized by knownmethods. Unless otherwise reported, all ratios are by weight percent.

The following abbreviations are used to describe the examples:

° C.: degrees Centigrade

cm: centimeter

LED: light emitting diode

mL: milliliter

mg: milligram

mm: millimeter

MPa: megaPascal

MW: molecular weight

nm: nanometer

rpm: revolutions per minute

T_(g): glass transition temperature

UV: ultraviolet

Abbreviations for the materials used in the examples are as follows:

-   CGI-90: Photolatert base obtained from BASF, Ludwigshafen, Germany-   CPP: Calcium peroxide powder, obtained under the trade designation    “IXPER 75C” from Solvay Chemicals, Inc., Houston, Tex.-   CPQ: Camphorquinone, a photosensitizer obtained from Sigma-Aldrich    Company.-   DABCO: A 33% by weight solution of 1,4-Diazabicyclo[2.2.2]octane in    dipropylene glycol, obtained under the trade designation “DABCO    33-LV” from Air Products & Chemicals, Inc., Allentown, Pa.-   DEA: 9,10-diethoxyanthracene, a photosensitizer obtained from Alfa    Aesa, Ward Hill, Mass.-   DMDO: 1,8-Dimercapto-3,6-dioxaoctane, obtained from Arkena, Inc.,    King of Prussia, Pa.-   DVE-3: Triethyleneglycol divinylether, obtained under the trade    designation “RAPI-CURE DVE-3” from Ashland Specialty Ingredients,    Wilmington, Del.-   E-8220: A diglycidylether of bisphenol F, obtained under the trade    designation “EPALLOY 8220” from Emerald Performance Materials, LLC,    Cuyahoga Falls, Ohio.-   FERBAM: Ferric dimethyldithiocarbamate, a fungicide, obtained under    the trade designation “FERBAM D1267” from TCI America Portland,    Oreg.-   G-12: A liquid polysulfide resin, mol. wt. 4,000, obtained under the    trade designation “THIOPLAST G12” from Akzo Nobel Functional    Chemicals GmbH.-   GE-23: A diepoxidized polyglycol obtained under the trade    designation “ERISYS GE-23” from Emerald Performance Materials, LLC.-   GE-30: Trimethylolpropane triglycidylether, obtained under the trade    designation “ERISYS GE-30” from Emerald Performance Materials    Company.-   IPA: Isopropyl alcohol.-   ITX: Isopropylthioxanthone, a photosensitizer obtained from    Sigma-Aldrich Company.-   TAC: Triallylcyanurate, obtained from Sartomer, Inc., Exton, Pa.-   TMG: Tetramethyl guanidine, obtained from Sigma-Aldrich Company.-   UPF: A surface-treated precipitated calcium carbonate, obtained    under the trade designation “ULTRA-PFLEX” from Minerals    Technologies, Inc. New York, N.Y.-   VAZO-52: 2,2′-azobis(2,4-dimethyl-pentanenitrile), obtained under    the trade designation “VAZO 52” from E.I. du Dupont de Nemours and    Company, Wilmington, Del.-   VAZO-67: 2,2′-azobis(2-methylbutyronitrile), obtained under the    trade designation “VAZO-67” from E.I. du Dupont de Nemours and    Company.-   LP-33: A liquid polysulfide polymer, obtained under the trade    designation “THIOKOL LP-33” from Toray Fine Chemicals Co., Ltd.,    Urayasu, Japan.-   AC-380A: Part A of a two-part polysulfide-based, manganese cured,    sealant, obtained under the trade designation “AEROSPACE SEALANT    AC-380 CLASS B-½” from 3M Company, St. Paul, Minn.-   AC-380B: Part B of a two-part polysulfide-based, manganese cured,    sealant, obtained under the trade designation “AEROSPACE SEALANT    AC-380 CLASS B-½” from 3M Company.-   AC-1: A thiol terminated polythioether oligomer with the equivalent    weight of 1458 was synthesized as follows. Into a 12-liter round    bottom flask equipped with an air-driven stirrer, thermometer, and a    dropping funnel, was added 4,706 grams (25.8 moles) DMDO and 999    grams (3.0 moles) E-8220 at 21° C. 1.7 grams DABCO (0.02 weight    percent) was mixed in as a catalyst. The system was flushed with    nitrogen, then mixed and heated for four hours at between 60-70° C.    150 grams (0.6 mole) TAC was added along with approximate 0.4 grams    VAZO-67. The material was mixed and heated at approximately 60° C.    for 3 hours. 3,758 grams (18.6 moles) DVE-3 was then added drop-wise    to the flask over 4 hours, keeping the temperature between 60-70° C.    An additional 1.2 grams VAZO-67 was then added in approximately 0.4    gram increments over approximately 8 hours. The temperature was then    raised to 100° C. and the material degassed for approximately 1    hour. The resultant polythioether was approximately 3200 MW with 2.2    functionality.    -   AC-2: A thiol terminated polythioether oligomer with the        equivalent weight of 283 was synthesized as follows. Into a        250-millliter round bottom flask equipped with an air-driven        stirrer, thermometer, and a dropping funnel, was added 128 grams        (0.7 moles) DMDO. The system was flushed with nitrogen, then        mixed and heated for 1 hour at between 55-60° C. 0.03 grams        VAZO-52 was then added and dissolved. 71 grams (0.4 moles) DVE-3        was then added drop-wise to the flask over 30 minutes, keeping        the temperature between 55-65° C. 0.1 gram VAZO-52 was added and        the material was allowed to stir for three hours. The        temperature was then raised to 100° C. and the material degassed        for approximately 1 hour. The resultant polythioether was        approximately 567 MW with 2.0 functionality.-   AC-3: A thiol terminated polythioether oligomer with the equivalent    weight of 476 was synthesized as follows. Into a 250-millliter round    bottom flask equipped with an air-driven stirrer, thermometer, and a    dropping funnel, was added 115 grams (0.6 moles) DMDO. The system    was flushed with nitrogen, then mixed and heated for 1 hour at    between 55-60° C. 0.03 grams VAZO-52 was then added and dissolved.    85 grams (0.4 moles) DVE-3 was then added drop-wise to the flask    over 30 minutes, keeping the temperature between 55-65° C. Another    0.03 gram VAZO-52 was added and the material was allowed to stir for    three hours. The temperature was then raised to 100° C. and the    material degassed for approximately 1 hour. The resultant    polythioether was approximately 951 MW with 2.0 functionality.

Mixture 1 (M-1)

A 20 mL amber glass vial was charged with 0.109 grams “CGI-90”photolatert base at 21° C. To this was added 4.000 grams DMDO, 3.906grams GE-23 and 3.028 grams GE-30. The vial was then sealed and placedon a laboratory roller mill for 2 hours at 25 rpm until the componentswere dissolved.

Mixtures 2-6 (M-2- M-6)

The procedure generally described for preparing Mixture M-1 wasrepeated, wherein the components were added to an amber vial in thesequence and quantities listed in Table 1.

Mixture 7 (M-7)

The procedure generally described for preparing Mixture M-6 wasrepeated, wherein the conterts of the vial was transferred to a jar,0.022 grams DABCO was added and the mixture homogeneously dispersed bymeans of a high speed mixer at 2,000 rpm for 1 minute at 21° C.

TABLE 1 Composition (grams) mixture CGI-90 ITX CPQ DMDO GE-23 GE-30DABCO M-1 0.109 0 0 4.000 3.906 3.028 0 M-2 0.109 0.109 0 4.000 3.9063.028 0 M-3 0.328 0 0 4.000 3.906 3.028 0 M-4 0.328 0.328 0 4.000 3.9063.028 0 M-5 0.328 0.547 0 4.000 3.906 3.028 0 M-6 0.328 0 0.328 4.0003.906 3.028 0 M-7 0.328 0 0.328 4.000 3.906 3.028 0.022Mixtures with Oligomer 1 (O-1)

A 20 mL amber glass vial was charged with 0.328 grams “CGI-90”photolatert base and an equal quantity of CPQ at 21° C. To this wasadded 10.000 grams AC-1 and 0.947 grams GE-30. The vial was then sealedand placed on a laboratory roller mill for 2 hours at 25 rpm until theCGI-90 and the CPQ were dissolved. The mixture was then transferred toan opaque jar, to which 0.274 grams DABCO was added, and the mixturehomogenously dispersed by means of a high speed mixer at 2,000 rpm for 1minute at 21° C.

Mixtures with Oligomers 2-10 (O-2-Ex. O-10)

The procedure generally described for preparing Mixtures with OligomerO-1 was repeated, wherein the components were added to an amber vial inthe sequence and quantities listed in Table 2. With respect to oligomersO-6, O-9 and O-10, the UPF was dispersed along with the DABCO.

TABLE 2 Composition (grams) Mixture CGI-90 CPQ ITX DEA AC-1 AC-2 AC-3GE-30 E-8220 DABCO UPF O-1 0.328 0.328 0 0 10.000 0 0 0.947 0 0.274 0O-2 0.328 0.328 0 0 10.000 0 0 0.947 0 0.033 0 O-3 0.328 0.328 0 010.000 0 0 0.947 0 0.547 0 O-4 0.312 0.312 0 0 0 7.000 0 3.413 0 0.031 0O-5 0.310 0.310 0 0 0 0 8.000 2.309 0 0.031 0 O-6 0.328 0.328 0 0 10.0000 0 0.947 0 0.033 3.284 O-7 0.328 0 0.328 0 10.000 0 0 0.947 0 0.033 0O-8 0.328 0 0 0.328 10.000 0 0 0.947 0 0.033 0 O-9 0.323 0 0.780 010.000 0 0 0 1.139 0.033 3.342 O-10 0.323 0 0.780 0 10.000 0 0 0 1.1390.033 5.570

Evaluations Exposure Source

LED: A 455 nm LED, model CT-2000, obtained from Clearstone Technologies,Inc., Hopkins, Minn.

H-Lamp: A mercury UV lamp, model F-600, obtained from Heraeus Holding,GmbH, Hanau, Germany.

Exposure Cure Time

The time, in minutes at 21° C., for the composition to fully cure in asilicone rubber mold when continuously exposed to either the LED or theH-lamp at a distance of 2.54 cm. Mold dimensions were 2.54 by 2.54 cm by2.54 mm, and 2.54 by 2.54 cm by 0.76 mm, for the LED and H-lampexposures, respectively.

Working Time

The time, in hours at 21° C., for the composition in the amber vial togel and become unusable.

Catalyst Cure Time

The time, in hours at 21° C., for the composition to fully cure in a2.54 by 2.54 cm by 2.54 mm silicone rubber mold without the LED orH-lamp exposure.

Curing results for the thiol epoxy monomers and thiol epoxy oligomersare listed in Table 3 and Table 4, respectively.

Tensile Strength

The composition was transferred to a 7.12 by 1.27 cm by 2.54 mm siliconrubber mold laminated in between a glass slide and a polyester releaseliner and cured at 21° C. by (a) exposure to the LED, at a distance of2.54 cm, for 2 minutes through the glass slide, followed by 1 minutethrough the release liner, or (b) catalyst cured for 24 hours. A sampleof the cured material was then die cut for the tensile strength testaccording to ASTM D-638V. Results are listed in Table 5.

TABLE 3 Cure Time Exposure Exposure Cure Working Time Catalyst CureMixture Source (minutes) (hours) (hours) M-1 H-Bulb Did not cure NotMeasured Not Measured M-2 H-Bulb Did not cure Not Measured Not MeasuredM-3 H-Bulb 2.0 Not Measured Not Measured M-4 H-Bulb 1.25 Not MeasuredNot Measured M-4 LED 2.5 Not Measured Not Measured M-5 LED 2.0 NotMeasured Not Measured M-6 LED 2.0 Not Measured Not Measured M-7 LED2.0 >2 Approx. 8

TABLE 4 Cure Time Exposure Exposure Cure Working Time Catalyst CureMixture Source (minutes) (hours) (hours) O-1 LED Not Measured Approx.3.5 Not Measured O-2 LED 1.5 Approx. 2.0 Approx. 8 O-3 LED Not Measured<1   Not Measured O-4 LED 1.5 Approx. 1.5 Not Measured O-5 LED 2.0 >2.0 Not Measured O-6 LED 2.0 Approx. 3.5 Not Measured O-7 LED 1.0 Approx.2.5 Not Measured O-8 LED 7.5 Approx. 2.0 Not Measured O-9 LED 1.5 >1.7510-16  O-10 LED 1.5 >1.75 10-16

TABLE 5 Elongation Tensile Strength Tg Oligomer Cure Type (%) (Mpa) (°C.) O-9  LED 355 1.78 Not Measured O-9  Catalyst 453 1.78 Not MeasuredO-10 LED 661 2.77 −53 O-10 Catalyst 900 3.42 Not Measured

Sprayable Catalyst A

A 20 mL amber glass vial was charged with 0.7208 grams “CGI-90”photolatert base, 0.7191 grams CPQ and 3.0932 grams IPA at 21° C. Themixture was vortex mixed until the “CGI-90” photolatert base and CPQwere completely dissolved. The mixture was then transferred to anaerosol sprayer.

Sprayable Catalyst B

A 20 mL amber glass vial was charged with 1.0 gram “CGI-90” photolatertbase, 1.0 gram ITX and 7.0 grams IPA at 21° C. The mixture was vortexmixed until the “CGI-90” photolatert base and ITX were completelydissolved. The mixture was then transferred to an aerosol sprayer.

Curable Sealant 1

A 20 mL amber glass vial was charged with 1.0 gram GE-30, 0.5 grams“CGI-90” photolatert base and 0.5 grams ITX at 21° C. The vial was thensealed and placed on a laboratory roller mill for 2 hours at 25 rpmuntil the “CGI-90” photolatert base was dissolved. The conterts of thevial were then transferred to a plastic jar and 10.0 grams AC-380Amanually mixed into the composition by means of a spatula.

Curable Sealants 1A and 1B

20 grams AC-380A was manually mixed with 2.0 grams GE-30 at 21° C. in aplastic jar by means of a spatula. The curable composition was thendivided into equal parts, 1-A and 1-B.

Curable Sealant 2

A 20 mL amber glass vial was charged with 1.0 gram GE-30, 0.5 grams“CGI-90” photolatert base and 0.5 grams ITX at 21° C. The vial was thensealed and placed on a laboratory roller mill for 2 hours at 25 rpmuntil the “CGI-90” photolatert base was dissolved. The conterts of thevial were then transferred to a plastic jar and 10.0 grams AC-380A and0.1 grams AC-380B manually mixed into the composition by means of aspatula.

Curable Sealants 2A and 2B

20 grams AC-380A was manually mixed with 2.0 grams GE-30 and 0.2 gramsAC-380B at 21° C. in a plastic jar by means of a spatula. The curablecomposition was then divided into equal parts, 2-A and 2-B.

Curable Sealants 3-5

The procedure generally described for preparing Curable Composition 2was repeated, according to the quantities listed in Table 6.

Curable Sealants 3A and 3B through 5A and 5B

The procedure generally described for preparing Curable Compositions 2Aand 2B was repeated, according to the quantities listed in Table 6.

Curable Sealant 6

A 20 mL amber glass vial was charged with 8.0 grams LP-33, 2.0 gramGE-30, 0.5 grams “CGI-90” photolatert base and 0.5 grams ITX at 21° C.The vial was then sealed and placed on a laboratory roller mill for 2hours at 25 rpm until the “CGI-90” photolatert base was dissolved.

Curable Sealant 7

A 20 mL amber glass vial was charged with 8.0 grams LP-33, 2.0 gramGE-30, 0.5 grams “CGI-90” photolatert base and 0.5 grams ITX at 21° C.The vial was then sealed and placed on a laboratory roller mill for 2hours at 25 rpm until the “CGI-90” photolatert base was dissolved. Theconterts of the vial were then transferred to a plastic jar and 0.1 gramAC-380B was manually mixed into the composition by means of a spatula.

Curable Sealants 8-9

The procedure generally described for preparing Curable Composition 7was repeated, according to the quantities listed in Table 6.

TABLE 6 Curable Components (grams) Sealant AC-380A GE-30 AC-380B LP-33CGI-90 ITX 1 10.0 1.0 0 0 0.5 0.5 1A 10.0 1.0 0 0 0 0 1B 10.0 1.0 0 0 00 2 10.0 1.0 0.1 0 0.5 0.5 2A 10.0 1.0 0.1 0 0 0 2B 10.0 1.0 0.1 0 0 0 310.0 1.0 0.5 0 0.5 0.5 3A 10.0 1.0 0.5 0 0 0 3B 10.0 1.0 0.5 0 0 0 410.0 1.0 1.0 0 0.5 0.5 4A 10.0 1.0 1.0 0 0 0 4B 10.0 1.0 1.0 0 0 0 510.0 0 1.0 0 0.5 0.5 5A 10.0 0 1.0 0 0 0 5B 10.0 0 1.0 0 0 0 6 0 2.0 08.0 0.5 0.5 7 0 2.0 0.1 8.0 0.5 0.5 8 0 2.0 0.5 8.0 0.5 0.5 9 0 2.0 18.0 0.5 0.5

The compositions were transferred to 1.88 by 3.15 cm by 2.8 mm Teflon™molds and subjected to one of the following curing protocols using amodel CT2000 LED, obtained from Clearstone Technologies, Inc., Hopkins,Minn.

Examples 1 to 10

At least one of an adhesion promoter or wetting agent can be added toeach of Curable Sealants 1A-5A and 1B to 5B. The curable sealants canthen be applied to an aircraft component.

Sprayable Curing

Curable sealants 1A, 2A, 3A, 4A and 5A were evenly sprayed withapproximately 35 mg Sprayable

Catalyst A, allowed to dry for 1 minute at 21° C., then exposed to theLED, at 50% power, for 1 minute at a distance of 2.54 cm.

Curable compositions 1B, 2B, 3B, 4B and 5B were evenly sprayed withapproximately 35 mg Sprayable Catalyst B, then dried and exposed to theLED and as per the “A” compositions above.

The thickness of cured compositions are listed in Table 7.

TABLE 7 Cured Cured Thickness Example Composition (mm) 1 1A 0.23 2 1B0.24 3 2A 0.25 4 2B 0.25 5 3A 0.23 6 3B 0 7 4A >0.1 8 4B 0 9 5A 0 10 5B0

Examples 11 to 19

At least one of an adhesion promoter or wetting agent can be added toeach of Curable Sealants 1 to 9. The curable sealants can then beapplied to an aircraft component.

Curable Sealants 1-5 were exposed to the LED, at 50% power, for 1 minuteat a distance of 2.54 cm. A second series of curable compositions wereexposed for the same time and at the same distance at 100% LED power.

Curable Compositions 6-9 were cured in a similar fashion to compositions1-5, at 50 and 75% LED power levels.

Thickness of the cured compositions 1-5 and 6-9 are listed in Tables 8and 9, respectively.

TABLE 8 Cured Thickness (mm) @ 50% @ 100% Example LED Power LED Power 110.25 0.20 12 0.20 >0.1 13 0.24 >0.1 14 >0.1 Surface charred 15 0.30Surface charred

TABLE 9 Cured Thickness (mm) @ 50% @ 75% Example LED Power LED Power 160 2.45 17 0 1.09 18 0 1.63 19 0 Sample charred

A second amine can be added, such as DABCO, can be added to any one ofCurable Compositions 1, 1A, 1B, 2, 2A, 2B, 3, 3A, 3B, 4, 4A, 4B, 5, 5A,5B, and 6 to 9 to provide Examples 20 to 38. In these examples, thesecond time period can be shorter than the second time period achievedfor Examples 1 to 19.

Example 20

A sealant mixture was prepared as follows. 50 grams G-12, 35 grams UPF,5 grams CPP and 0.5 grams FERBAM were mixed under vacuum in a 200 mL cupfor 2 minutes, at 21° C. and 1,200 rpm, using a model “DAC 600.1 VAC-P”speed mixer, from Flack Tec, Inc., Landrum, S.C. To this was added 0.1grams TMG and mixing continued for another 30 seconds at 1,200 rpm untilthe sealant was homogeneous. The cup was removed from the mixer, thecover removed and the conterts flushed with nitrogen. The cup was againcovered and the sealant allowed to equilibrate at 25° C. for 2 hours.The sealant was then transferred to an open 8.8 by 3.1 by 0.25 cmsilicone rubber mold and exposed to an atmosphere of 50% relativehumidity at 25° C. After 24 hours the sealant had formed a durable,tack-free skin, approximately lmm thick, over a viscous liquid. After anadditional 168 hours exposure to this atmosphere the skin thicknessincreased had to 1.5 mm.

Various modifications and alterations of this disclosure may be made bythose skilled the art without departing from the scope and spirit of thedisclosure, and it should be understood that this invention is not to beunduly limited to the illustrative embodiments set forth herein.

1. A method of applying a sealant to an aircraft component, the methodcomprising: applying a curable sealant to a surface of the aircraftcomponent, wherein the curable sealant comprises at least one of anadhesion promoter or a wetting agent; and forming a non-tacky skin on anexposed portion of the curable sealant within a first time period whileallowing a portion of the curable sealant adjacent the surface of theaircraft component to be liquid for a second time period, wherein thesecond time period is at least twice the first time period, and whereinthe first time period and the second time period begin simultaneously.2. The method of claim 1, wherein the first time period is up to fourhours.
 3. The method of claim 1, wherein the second time period is atleast ten times the first time period, or wherein the second time periodis sufficient to allow for at least one of the adhesion promoter orwetting agent to migrate to the surface of the aircraft component. 4.The method of claim 1, wherein the curable sealant is applied to a seamor joint between portions of aircraft skin.
 5. The method of claim 1,wherein the curable sealant is applied to at least one of an aircraftfastener, an aircraft window, an aircraft access panel, a fuselageprotrusion, or an aircraft fuel tank.
 6. The method of claim 1, whereinthe curable sealant comprises a polythiol comprising more than one thiolgroup.
 7. The method of claim 6, wherein the polythiol is monomeric. 8.The method of claim 6, wherein the polythiol is an oligomeric orpolymeric polythioether or polysulfide.
 9. The method of claim 6,wherein the curable sealant comprises a polyepoxide comprising more thanone epoxide group.
 10. The method of claim 6, wherein the curablesealant comprises a Michael acceptor comprising more than one Michaelacceptor group.
 11. The method of claim 9 or 10, wherein the curablesealant further comprises a photolatert base.
 12. The method of claim 9or 10, further comprising applying a solution comprising a photolatertbase catalyst to the exposed portion of the curable sealant beforeexposing the exposed portion of the curable sealant to actinicradiation.
 13. The method of claim 1, wherein forming a non-tacky skinon an exposed portion of the curable sealant comprises exposing theexposed portion of the curable sealant to actinic radiation.
 14. Themethod of claim 6, wherein the curable sealant comprises at least oneunsaturated compound comprising more than one carbon-carbon double bond,carbon-carbon triple bond, or a combination thereof and aphotoinitiator, and wherein forming a non-tacky skin on an exposedportion of the curable sealant comprises exposing the exposed portion ofthe curable sealant to actinic radiation.
 15. The method of claim 6,wherein the curable sealant comprises at least one of anoxygen-activated curing agent or a moisture-activated curing agent. 16.The method of claim 6, wherein the polythiol is an oligomer or polymerprepared from components comprising a dithiol and a diene or divinylether.
 17. The method of claim 1, wherein the adhesion promotercomprises at least one of a phenolic resin or an amino-, mercapto-, orepoxy-functional silane.
 18. The method of claim 1, wherein the wettingagent comprises at least one of a silicone, fluorinated, or hydrocarbonsurfactant.
 19. The method of claim 1, wherein the second time period isat least one hundred times the first time period.
 20. The method ofclaim 1, wherein the first time period is up to one minute.